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Candida albicans is a pathogenic fungus that causes systemic infections and mortality in immunosuppressed individuals. We previously showed that deacetylation of histone H3 lysine 56 by Hst3 is essential for C. albicans viability. Hst3 is a fungal-specific NAD+ -dependent protein deacetylase of the sirtuin family. In vivo, supraphysiological concentrations of nicotinamide (NAM) are required for Hst3 inhibition and cytotoxicity. This underscores the importance of identifying mechanisms by which C. albicans can modulate intracellular NAM concentrations. For the first time in a pathogenic fungus, we combine genetics, heavy isotope labeling, and targeted quantitative metabolomics to identify genes, pathways, and mechanisms by which C. albicans can reduce the cytotoxicity of high NAM concentrations. We discovered three distinct fates for supraphysiological NAM concentrations. First, upon transient exposure to NAM, high intracellular NAM concentrations rapidly return near the physiological levels observed in cells that are not exposed to NAM. Second, during the first step of a fungal-specific NAM salvage pathway, NAM is converted into nicotinic acid, a metabolite that cannot inhibit the sirtuin Hst3. Third, we provide evidence that NAM enters the NAD+ metabolome through a NAM exchange reaction that contributes to NAM-mediated inhibition of sirtuins. However, in contrast to the other fates of NAM, the NAM exchange reaction cannot cause a net decrease in the intracellular concentration of NAM. Therefore, this reaction cannot enhance resistance to NAM. In summary, we demonstrate that C. albicans possesses at least two mechanisms to attenuate the cytotoxicity of pharmacological NAM concentrations. It seems likely that those two mechanisms of resistance to cytotoxic NAM concentrations are conserved in many other pathogenic fungi.

The identification of structural variations in genomes using next-generation sequencing approaches greatly facilitates the study of genetic and genomic diseases. The data generated using these approaches also provide interesting new means to examine DNA repair, recombination, and replication to better understand sources of genomic instability. To better utilize this data, we developed SCARR (Systematic Combination of Alignments to Recreate Rearrangements) to identify DNA rearrangements, and used it to examine the occurrence of orientation-reversing events in human and budding yeast genomes. SCARR exceeds the sensitivity of previous genome sequencing approaches, and identifies rearrangements genome-wide with base-pair resolution, which helps provide insights into the mechanisms involved in their formation. We find that short-range orientation-reversing events occur at high rates in both human and yeast genomes. We quantified these rearrangements in yeast strains lacking various DNA repair factors, and propose that these short-range events often occur through template-switching events within a replication fork. We hypothesize that this mechanism may act as an error-prone alternative to fork reversal to restart stalled replication forks.

During DNA replication, proliferating cell nuclear antigen (PCNA) adopts a ring-shaped structure to promote processive DNA synthesis, acting as a sliding clamp for polymerases. Known posttranslational modifications function at the outer surface of the PCNA ring to favor DNA damage bypass. Here, we demonstrate that acetylation of lysine residues at the inner surface of PCNA is induced by DNA lesions. We show that cohesin acetyltransferase Eco1 targets lysine 20 at the sliding surface of the PCNA ring in vitro and in vivo in response to DNA damage. Mimicking constitutive acetylation stimulates homologous recombination and robustly suppresses the DNA damage sensitivity of mutations in damage tolerance pathways. In comparison to the unmodified trimer, structural differences are observed at the interface between protomers in the crystal structure of the PCNA-K20ac ring. Thus, acetylation regulates PCNA sliding on DNA in the presence of DNA damage, favoring homologous recombination linked to sister-chromatid cohesion.

Oncogenic transcription factors are commonly activated in acute leukemias and subvert normal gene expression networks to reprogram hematopoietic progenitors into preleukemic stem cells, as exemplified by LIM-only 2 (LMO2) in T-cell acute lymphoblastic leukemia (T-ALL). Whether or not these oncoproteins interfere with other DNA-dependent processes is largely unexplored. Here, we show that LMO2 is recruited to DNA replication origins by interaction with three essential replication enzymes: DNA polymerase delta (POLD1), DNA primase (PRIM1), and minichromosome 6 (MCM6). Furthermore, tethering LMO2 to synthetic DNA sequences is sufficient to transform these sequences into origins of replication. We next addressed the importance of LMO2 in erythroid and thymocyte development, two lineages in which cell cycle and differentiation are tightly coordinated. Lowering LMO2 levels in erythroid progenitors delays G1-S progression and arrests erythropoietin-dependent cell growth while favoring terminal differentiation. Conversely, ectopic expression in thymocytes induces DNA replication and drives these cells into cell cycle, causing differentiation blockade. Our results define a novel role for LMO2 in directly promoting DNA synthesis and G1-S progression.

Chromatin Assembly Factor I (CAF-I) plays a key role in the replication-coupled assembly of nucleosomes. It is expected that its function is linked to the regulation of the cell cycle, but little detail is available. Current models suggest that CAF-I is recruited to replication forks and to chromatin via an interaction between its Cac1p subunit and the replication sliding clamp, PCNA, and that this interaction is stimulated by the kinase CDC7. Here we show that another kinase, CDC28, phosphorylates Cac1p on serines 94 and 515 in early S phase and regulates its association with chromatin, but not its association with PCNA. Mutations in the Cac1p-phosphorylation sites of CDC28 but not of CDC7 substantially reduce the in vivo phosphorylation of Cac1p. However, mutations in the putative CDC7 target sites on Cac1p reduce its stability. The association of CAF-I with chromatin is impaired in a cdc28–1 mutant and to a lesser extent in a cdc7–1 mutant. In addition, mutations in the Cac1p-phosphorylation sites by both CDC28 and CDC7 reduce gene silencing at the telomeres. We propose that this phosphorylation represents a regulatory step in the recruitment of CAF-I to chromatin in early S phase that is distinct from the association of CAF-I with PCNA. Hence, we implicate CDC28 in the regulation of chromatin reassembly during DNA replication. These findings provide novel mechanistic insights on the links between cell-cycle regulation, DNA replication and chromatin reassembly.

In Saccharomyces cerevisiae, histone H3 lysine 56 acetylation (H3K56ac) is a modification of new H3 molecules deposited throughout the genome during S-phase. H3K56ac is removed by the sirtuins Hst3 and Hst4 at later stages of the cell cycle. Previous studies indicated that regulated degradation of Hst3 plays an important role in the genome-wide waves of H3K56 acetylation and deacetylation that occur during each cell cycle. However, little is known regarding the mechanism of cell cycle-regulated Hst3 degradation. Here, we demonstrate that Hst3 instability in vivo is dependent upon the ubiquitin ligase SCF(Cdc4) and that Hst3 is phosphorylated at two Cdk1 sites, threonine 380 and threonine 384. This creates a diphosphorylated degron that is necessary for Hst3 polyubiquitylation by SCF(Cdc4). Mutation of the Hst3 diphospho-degron does not completely stabilize Hst3 in vivo, but it nonetheless results in a significant fitness defect that is particularly severe in mutant cells treated with the alkylating agent methyl methanesulfonate. Unexpectedly, we show that Hst3 can be degraded between G2 and anaphase, a window of the cell cycle where Hst3 normally mediates genome-wide deacetylation of H3K56. Our results suggest an intricate coordination between Hst3 synthesis, genome-wide H3K56 deacetylation by Hst3, and cell cycle-regulated degradation of Hst3 by cyclin-dependent kinases and SCF(Cdc4).

The MYST protein lysine acetyltransferases are evolutionarily conserved throughout eukaryotes and acetylate proteins to regulate diverse biological processes including gene regulation, DNA repair, cell-cycle regulation, stem cell homeostasis and development. Here, we demonstrate that MYST protein acetyltransferase activity requires active site lysine autoacetylation. The X-ray crystal structures of yeast Esa1 (yEsa1/KAT5) bound to a bisubstrate H4K16CoA inhibitor and human MOF (hMOF/KAT8/MYST1) reveal that they are autoacetylated at a strictly conserved lysine residue in MYST proteins (yEsa1-K262 and hMOF-K274) in the enzyme active site. The structure of hMOF also shows partial occupancy of K274 in the unacetylated form, revealing that the side chain reorients to a position that engages the catalytic glutamate residue and would block cognate protein substrate binding. Consistent with the structural findings, we present mass spectrometry data and biochemical experiments to demonstrate that this lysine autoacetylation on yEsa1, hMOF and its yeast orthologue, ySas2 (KAT8) occurs in solution and is required for acetylation and protein substrate binding in vitro. We also show that this autoacetylation occurs in vivo and is required for the cellular functions of these MYST proteins. These findings provide an avenue for the autoposttranslational regulation of MYST proteins that is distinct from other acetyltransferases but draws similarities to the phosphoregulation of protein kinases.

SummaryYeast Rtt109 promotes nucleosome assembly and genome stability by acetylating K9, K27, and K56 of histone H3 through interaction with either of two distinct histone chaperones, Vps75 or Asf1. We report the crystal structure of an Rtt109-AcCoA/Vps75 complex revealing an elongated Vps75 homodimer bound to two globular Rtt109 molecules to form a symmetrical holoenzyme with a ∼12 Å diameter central hole. Vps75 and Rtt109 residues that mediate complex formation in the crystals are also important for Rtt109-Vps75 interaction and H3K9/K27 acetylation both in vitro and in yeast cells. The same Rtt109 residues do not participate in Asf1-mediated Rtt109 acetylation in vitro or H3K56 acetylation in yeast cells, demonstrating that Asf1 and Vps75 dictate Rtt109 substrate specificity through distinct mechanisms. These studies also suggest that Vps75 binding stimulates Rtt109 catalytic activity by appropriately presenting the H3-H4 substrate within the central cavity of the holoenzyme to promote H3K9/K27 acetylation of new histones before deposition.

Cdc7 phosphorylates Rad18 to integrate S phase progression with postreplication DNA repair, ensuring genome stability., The E3 ubiquitin ligase Rad18 guides DNA Polymerase eta (Polη) to sites of replication fork stalling and mono-ubiquitinates proliferating cell nuclear antigen (PCNA) to facilitate binding of Y family trans-lesion synthesis (TLS) DNA polymerases during TLS. However, it is unclear exactly how Rad18 is regulated in response to DNA damage and how Rad18 activity is coordinated with progression through different phases of the cell cycle. Here we identify Rad18 as a novel substrate of the essential protein kinase Cdc7 (also termed Dbf4/Drf1-dependent Cdc7 kinase [DDK]). A serine cluster in the Polη-binding motif of Rad18 is phosphorylated by DDK. Efficient association of Rad18 with Polη is dependent on DDK and is necessary for redistribution of Polη to sites of replication fork stalling. This is the first demonstration of Rad18 regulation by direct phosphorylation and provides a novel mechanism for integration of S phase progression with postreplication DNA repair to maintain genome stability.

Chromatin assembly factor 1 (CAF-1) deposits histones H3 and H4 rapidly behind replication forks through an interaction with the proliferating cell nuclear antigen (PCNA), a DNA polymerase processivity factor that also binds to a number of replication enzymes and other proteins that act on nascent DNA. The mechanisms that enable CAF-1 and other PCNA-binding proteins to function harmoniously at the replication fork are poorly understood. Here we report that the large subunit of human CAF-1 (p150) contains two distinct PCNA interaction peptides (PIPs). The N-terminal PIP binds strongly to PCNA in vitro but, surprisingly, is dispensable for nucleosome assembly and only makes a modest contribution to targeting p150 to DNA replication foci in vivo. In contrast, the internal PIP (PIP2) lacks one of the highly conserved residues of canonical PIPs and binds weakly to PCNA. Surprisingly, PIP2 is essential for nucleosome assembly during DNA replication in vitro and plays a major role in targeting p150 to sites of DNA replication. Unlike canonical PIPs, such as that of p21, the two p150 PIPs are capable of preferentially inhibiting nucleosome assembly, rather than DNA synthesis, suggesting that intrinsic features of these peptides are part of the mechanism that enables CAF-1 to function behind replication forks without interfering with other PCNA-mediated processes.

(2009). Histone acetylation floods the human cell cycle and DNA damage response. Cell Cycle: Vol. 8, No. 12, pp. 1818-1822.

Sulfation and phosphorylation are post-translational modifications imparting an isobaric 80-Da addition on the side chain of serine, threonine, or tyrosine residues. These two post-translational modifications are often difficult to distinguish because of their similar MS fragmentation patterns. Targeted MS identification of these modifications in specific proteins commonly relies on their prior separation using gel electrophoresis and silver staining. In the present investigation, we report a potential pitfall in the interpretation of these modifications from silver-stained gels due to artifactual sulfation of serine, threonine, and tyrosine residues by sodium thiosulfate, a commonly used reagent that catalyzes the formation of metallic silver deposits onto proteins. Detailed MS analyses of gel-separated protein standards and Escherichia coli cell extracts indicated that several serine, threonine, and tyrosine residues were sulfated using silver staining protocols but not following Coomassie Blue staining. Sodium thiosulfate was identified as the reagent leading to this unexpected side reaction, and the degree of sulfation was correlated with increasing concentrations of thiosulfate up to 0.02%, which is typically used for silver staining. The significance of this artifact is discussed in the broader context of sulfation and phosphorylation site identification from in vivo and in vitro experiments.

Deacetylation of histone H3 K56, regulated by the sirtuins Hst3p and Hst4p, is critical for maintenance of genomic stability. However, the physiological consequences of a lack of H3 K56 deacetylation are poorly understood. Here we show that cells lacking Hst3p and Hst4p, in which H3 K56 is constitutively hyperacetylated, exhibit hallmarks of spontaneous DNA damage, such as activation of the checkpoint kinase Rad53p and upregulation of DNA-damage inducible genes. Consistently, hst3 hst4 cells display synthetic lethality interactions with mutations that cripple genes involved in DNA replication and DNA double-strand break (DSB) repair. In most cases, synthetic lethality depends upon hyperacetylation of H3 K56 because it can be suppressed by mutation of K56 to arginine, which mimics the nonacetylated state. We also show that hst3 hst4 phenotypes can be suppressed by overexpression of the PCNA clamp loader large subunit, Rfc1p, and by inactivation of the alternative clamp loaders CTF18, RAD24, and ELG1. Loss of CTF4, encoding a replisome component involved in sister chromatid cohesion, also suppresses hst3 hst4 phenotypes. Genetic analysis suggests that CTF4 is a part of the K56 acetylation pathway that converges on and modulates replisome function. This pathway represents an important mechanism for maintenance of genomic stability and depends upon proper regulation of H3 K56 acetylation by Hst3p and Hst4p. Our data also suggest the existence of a precarious balance between Rfc1p and the other RFC complexes and that the nonreplicative forms of RFC are strongly deleterious to cells that have genomewide and constitutive H3 K56 hyperacetylation.

Summary RbAp46 and RbAp48 (pRB-associated proteins p46 and p48, also known as RBBP7 and RBBP4, respectively) are highly homologous histone chaperones that play key roles in establishing and maintaining chromatin structure. We report here the crystal structure of human RbAp46 bound to histone H4. RbAp46 folds into a seven-bladed β propeller structure and binds histone H4 in a groove formed between an N-terminal α helix and an extended loop inserted into blade six. Surprisingly, histone H4 adopts a different conformation when interacting with RbAp46 than it does in either the nucleosome or in the complex with ASF1, another histone chaperone. Our structural and biochemical results suggest that when a histone H3/H4 dimer (or tetramer) binds to RbAp46 or RbAp48, helix 1 of histone H4 unfolds to interact with the histone chaperone. We discuss the implications of our findings for the assembly and function of RbAp46 and RbAp48 complexes.

In Saccharomyces cerevisiae, acetylation of lysine 56 (Lys-56) in the globular domain of histone H3 plays an important role in response to genotoxic agents that interfere with DNA replication. However, the regulation and biological function of this modification are poorly defined in other eukaryotes. Here we show that Lys-56 acetylation in Schizosaccharomyces pombe occurs transiently during passage through S-phase and is normally removed in G(2). Genotoxic agents that cause DNA double strand breaks during replication elicit a delay in deacetylation of histone H3 Lys-56. In addition, mutant cells that cannot acetylate Lys-56 are acutely sensitive to genotoxic agents that block DNA replication. Moreover, we show that Spbc342.06cp, a previously uncharacterized open reading frame, encodes the functional homolog of S. cerevisiae Rtt109, and that this protein acetylates H3 Lys-56 both in vitro and in vivo. Altogether, our results indicate that both the regulation of histone H3 Lys-56 acetylation by its histone acetyltransferase and histone deacetylase and its role in the DNA damage response are conserved among two distantly related yeast model organisms.

Protein post-translational modifications (PTMs) play important roles in the control of various biological processes including protein–protein interactions, epigenetics and cell cycle regulation. Mass spectrometry-based proteomics approaches enable comprehensive identification and quantitation of numerous types of PTMs. However, the analysis of PTMs is complicated by the presence of indistinguishable co-eluting isomeric peptides that result in composite spectra with overlapping features that prevent the identification of individual components. In this study, we present Iso-PeptidAce, a novel software tool that enables deconvolution of composite MS/MS spectra of isomeric peptides based on features associated with their characteristic fragment ion patterns. We benchmark Iso-PeptidAce using dilution series prepared from mixtures of known amounts of synthetic acetylated isomers. We also demonstrate its applicability to different biological problems such as the identification of site-specific acetylation patterns in histones bound to chromatin assembly factor-1 and profiling of histone acetylation in cells treated with different classes of HDAC inhibitors., Deciphering patterns of histone modifications that modulate chromatin structure and function is important, but remains challenging. Here the authors describe a method to uncover patterns of site-specific histone acetylation by deconvolution of overlapping peptide isomer mass spectra.

In Saccharomyces cerevisiae, histone H3 lysine 56 acetylation (H3K56Ac) is present in newly synthesized histones deposited throughout the genome during DNA replication. The sirtuins Hst3 and Hst4 deacetylate H3K56 after S phase, and virtually all histone H3 molecules are K56 acetylated throughout the cell cycle in hst3∆ hst4∆ mutants. Failure to deacetylate H3K56 causes thermosensitivity, spontaneous DNA damage, and sensitivity to replicative stress via molecular mechanisms that remain unclear. Here we demonstrate that unlike wild-type cells, hst3∆ hst4∆ cells are unable to complete genome duplication and accumulate persistent foci containing the homologous recombination protein Rad52 after exposure to genotoxic drugs during S phase. In response to replicative stress, cells lacking Hst3 and Hst4 also displayed intense foci containing the Rfa1 subunit of the single-stranded DNA binding protein complex RPA, as well as persistent activation of DNA damage–induced kinases. To investigate the basis of these phenotypes, we identified histone point mutations that modulate the temperature and genotoxic drug sensitivity of hst3∆ hst4∆ cells. We found that reducing the levels of histone H4 lysine 16 acetylation or H3 lysine 79 methylation partially suppresses these sensitivities and reduces spontaneous and genotoxin-induced activation of the DNA damage-response kinase Rad53 in hst3∆ hst4∆ cells. Our data further suggest that elevated DNA damage–induced signaling significantly contributes to the phenotypes of hst3∆ hst4∆ cells. Overall, these results outline a novel interplay between H3K56Ac, H3K79 methylation, and H4K16 acetylation in the cellular response to DNA damage.

Several recent reports have identified lysine 56 (K56) as a novel site of acetylation in yeast histone H3. K56 acetylation is predicted to disrupt some of the histone-DNA interactions at the entry and exit points of the nucleosome core particle. This modification occurs in virtually all the newly synthesised histones that are deposited into chromatin during S-phase. Cells with mutations that block K56 acetylation show increased genome instability and hypersensitivity to genotoxic agents that interfere with replication. Removal of K56 acetylation takes place in the G2/M phase of the cell cycle and is dependent upon Hst3 and Hst4, two proteins that are related to the NAD+-dependent histone deacetylase Sir2. In response to DNA damage checkpoint activation during S-phase, expression of Hst3/Hst4 is delayed to extend the window of opportunity in which K56 acetylation can act in the DNA damage response. The high abundance of histone H3 K56 acetylation, its regulation and strategic location in the nucleosome core particle raise a number of fascinating issues that we discuss here.

Human transcriptional coactivator PC4 is a highly abundant multifunctional protein which plays diverse important roles in cellular processes, including transcription, replication, and repair. It is also a unique activator of p53 function. Here we report that PC4 is a bona fide component of chromatin with distinct chromatin organization ability. PC4 is predominantly associated with the chromatin throughout the stages of cell cycle and is broadly distributed on the mitotic chromosome arms in a punctate manner except for the centromere. It selectively interacts with core histones H3 and H2B; this interaction is essential for PC4-mediated chromatin condensation, as demonstrated by micrococcal nuclease (MNase) accessibility assays, circular dichroism spectroscopy, and atomic force microscopy (AFM). The AFM images show that PC4 compacts the 100-kb reconstituted chromatin distinctly compared to the results seen with the linker histone H1. Silencing of PC4 expression in HeLa cells results in chromatin decompaction, as evidenced by the increase in MNase accessibility. Knocking down of PC4 up-regulates several genes, leading to the G2/M checkpoint arrest of cell cycle, which suggests its physiological role as a chromatin-compacting protein. These results establish PC4 as a new member of chromatin-associated protein family, which plays an important role in chromatin organization.

Proliferating cells need to synthesize large amounts of histones to rapidly package nascent DNA into nucleosomes. This is a challenging task for cells because changes in rates of DNA synthesis lead to an accumulation of excess histones, which interfere with many aspects of DNA metabolism. In addition, cells need to ensure that histone variants are incorporated at the correct chromosomal location. Recent discoveries have highlighted the importance of regulated histone proteolysis in preventing both the accumulation of excess histones and the mis-incorporation of histone variants at inappropriate loci.

DNA breaks are extremely harmful lesions that need to be repaired efficiently throughout the genome. However, the packaging of DNA into nucleosomes is a significant barrier to DNA repair, and the mechanisms of repair in the context of chromatin are poorly understood. Here we show that lysine 56 (K56) acetylation is an abundant modification of newly synthesized histone H3 molecules that are incorporated into chromosomes during S phase. Defects in the acetylation of K56 in histone H3 result in sensitivity to genotoxic agents that cause DNA strand breaks during replication. In the absence of DNA damage, the acetylation of histone H3 K56 largely disappears in G2. In contrast, cells with DNA breaks maintain high levels of acetylation, and the persistence of the modification is dependent on DNA damage checkpoint proteins. We suggest that the acetylation of histone H3 K56 creates a favourable chromatin environment for DNA repair and that a key component of the DNA damage response is to preserve this acetylation.

HP1 family proteins are adaptor molecules, containing two related chromo domains that are required for chromatin packaging and gene silencing. Here we present the structure of the chromo shadow domain from mouse HP1beta bound to a peptide containing a consensus PXVXL motif found in many HP1 binding partners. The shadow domain exhibits a novel mode of peptide recognition, where the peptide binds across the dimer interface, sandwiched in a beta-sheet between strands from each monomer. The structure allows us to predict which other shadow domains bind similar PXVXL motif-containing peptides and provides a framework for predicting the sequence specificity of the others. We show that targeting of HP1beta to heterochromatin requires shadow domain interactions with PXVXL-containing proteins in addition to chromo domain recognition of Lys-9-methylated histone H3. Interestingly, it also appears to require the simultaneous recognition of two Lys-9-methylated histone H3 molecules. This finding implies a further complexity to the histone code for regulation of chromatin structure and suggests how binding of HP1 family proteins may lead to its condensation.

The human base excision repair machinery must locate and repair DNA base damage present in chromatin, of which the nucleosome core particle is the basic repeating unit. Here, we have utilized fragments of the Lytechinus variegatus 5S rRNA gene containing site-specific U:A base pairs to investigate the base excision repair pathway in reconstituted nucleosome core particles in vitro. The human uracil-DNA glycosylases, UNG2 and SMUG1, were able to remove uracil from nucleosomes. Efficiency of uracil excision from nucleosomes was reduced 3- to 9-fold when compared with naked DNA, and was essentially uniform along the length of the DNA substrate irrespective of rotational position on the core particle. Furthermore, we demonstrate that the excision repair pathway of an abasic site can be reconstituted on core particles using the known repair enzymes, AP-endonuclease 1, DNA polymerase beta and DNA ligase III. Thus, base excision repair can proceed in nucleosome core particles in vitro, but the repair efficiency is limited by the reduced activity of the uracil-DNA glycosylases and DNA polymerase beta on nucleosome cores.

Mechanisms contributing to the maintenance of heterochromatin in proliferating cells are poorly understood. We demonstrate that chromatin assembly factor 1 (CAF-1) binds to mouse HP1 proteins via an N-terminal domain of its p150 subunit, a domain dispensable for nucleosome assembly during DNA replication. Mutations in p150 prevent association with HP1 in heterochromatin in cells that are not in S phase and the formation of CAF-1-HP1 complexes in nascent chromatin during DNA replication in vitro. We suggest that CAF-1 p150 has a heterochromatin-specific function distinct from its nucleosome assembly function during S phase. Just before mitosis, CAF-1 p150 and some HP1 progressively dissociate from heterochromatin concomitant with histone H3 phosphorylation. The HP1 proteins reassociate with chromatin at the end of mitosis, as histone H3 is dephosphorylated.

Chromatin assembly factor 1 (CAF-1) assembles nucleosomes in a replication-dependent manner. The small subunit of CAF-1 (p48) is a member of a highly conserved subfamily of WD-repeat proteins. There are at least two members of this subfamily in both human (p46 and p48) and yeast cells (Hat2p, a subunit of the B-type H4 acetyltransferase, and Msi1p). Human p48 can bind to histone H4 in the absence of CAF-1 p150 and p60. p48, also a known subunit of a histone deacetylase, copurifies with a chromatin assembly complex (CAC), which contains the three subunits of CAF-1 (p150, p60, p48) and H3 and H4, and promotes DNA replication-dependent chromatin assembly. CAC histone H4 exhibits a novel pattern of lysine acetylation that overlaps with, but is distinct from, that reported for newly synthesized H4 isolated from nascent chromatin. Our data suggest that CAC is a key intermediate of the de novo nucleosome assembly pathway and that the p48 subunit participates in other aspects of histone metabolism.

Heterochromatin protein 1 (HP1β), a key component of condensed DNA, is strongly implicated in gene silencing and centromeric cohesion. Heterochromatin has been considered a static structure, stabilizing crucial aspects of nuclear organization and prohibiting access to transcription factors. We demonstrate here, by fluorescence recovery after photobleaching, that a green fluorescent protein–HP1β fusion protein is highly mobile within both the euchromatin and heterochromatin of ex vivo resting murine T cells. Moreover, T cell activation greatly increased this mobility, indicating that such a process may facilitate (hetero)chromatin remodeling and permit access of epigenetic modifiers and transcription factors to the many genes that are consequently derepressed.

Rtt109 is a fungal-specific histone acetyltransferase (HAT) that associates with either Vps75 or Asf1 to acetylate histone H3. Recent biochemical and structural studies suggest that site-specific acetylation of H3 by Rtt109 is dictated by the binding chaperone where Rtt109–Asf1 acetylates K56, while Rtt109–Vps75 acetylates K9 and K27. To gain further insights into the roles of Vps75 and Asf1 in directing site-specific acetylation of H3, we used quantitative proteomics to profile the global and site-specific changes in H3 and H4 during in vitro acetylation assays with Rtt109 and its chaperones. Our analyses showed that Rtt109–Vps75 preferentially acetylates H3 K9 and K23, the former residue being the major acetylation site. At high enzyme-to-substrate ratio, Rtt109 also acetylated K14, K18, K27 and to a lower extent K56 of histone H3. Importantly, this study revealed that in contrast to Rtt109–Vps75, Rtt109–Asf1 displayed a far greater site-specificity, with K56 being the primary site of acetylation. For the first time, we also report the acetylation of histone H4 K12 by Rtt109–Vps75, whereas Rtt109–Asf1 showed no detectable activity toward H4. This article is part of a Special Issue entitled: From protein structures to clinical applications.

Histone deacetylase inhibitors (HDACi) represent a promising avenue for cancer therapy. We applied mass spectrometry (MS) to determine the impact of clinically relevant HDACi on global levels of histone acetylation. Intact histone profiling revealed that the HDACi SAHA and MS-275 globally increased histone H3 and H4 acetylation in both normal diploid fibroblasts and transformed human cells. Histone H3 lysine 56 acetylation (H3K56ac) recently elicited much interest and controversy due to its potential as a diagnostic and prognostic marker for a broad diversity of cancers. Using quantitative MS, we demonstrate that H3K56ac is much less abundant than previously reported in human cells. Unexpectedly, in contrast to H3/H4 N-terminal tail acetylation, H3K56ac did not increase in response to inhibitors of each class of HDACs. In addition, we demonstrate that antibodies raised against H3K56ac peptides cross-react against H3 N-terminal tail acetylation sites that carry sequence similarity to residues flanking H3K56.

In Saccharomyces cerevisiae, histone H3 lysine 56 acetylation (H3K56ac) occurs in newly synthesized histones that are deposited throughout the genome during DNA replication. Defects in H3K56ac sensitize cells to genotoxic agents, suggesting that this modification plays an important role in the DNA damage response. However, the links between histone acetylation, the nascent chromatin structure, and the DNA damage response are poorly understood. Here we report that cells devoid of H3K56ac are sensitive to DNA damage sustained during transient exposure to methyl methanesulfonate (MMS) or camptothecin but are only mildly affected by hydroxyurea. We demonstrate that, after exposure to MMS, H3K56ac-deficient cells cannot complete DNA replication and eventually segregate chromosomes with intranuclear foci containing the recombination protein Rad52. In addition, we provide evidence that these phenotypes are not due to defects in base excision repair, defects in DNA damage tolerance, or a lack of Rad51 loading at sites of DNA damage. Our results argue that the acute sensitivity of H3K56ac-deficient cells to MMS and camptothecin stems from a failure to complete the repair of specific types of DNA lesions by recombination and/or from defects in the completion of DNA replication.

Oncogenic transcription factors are major drivers in acute leukemias. These oncogenes are believed to subvert normal cell identity via the establishment of gene expression programs that dictate cell differentiation and growth. The LMO2 oncogene, which is commonly activated in T-cell acute lymphoblastic leukemia (T-ALL), has a well-established function in transcription regulation. We and others previously demonstrated that LMO1 or LMO2 collaborate with the SCL transcription factor to activate a self-renewal program that converts non self-renewing progenitors into pre-leukemic stem cells. Here we demonstrate a non-transcriptional role of LMO2 in controlling cell fate by directly promoting DNA replication, a hitherto unrecognized mechanism that might also account for its oncogenic properties. To address the question whether LMO2 controls other functions via protein-protein interactions, we performed a proteome-wide screen for LMO2 interaction partners in Kit+ Lin- cells. In addition to known LMO2-interacting proteins such as LDB1 and to proteins associated with transcription, we unexpectedly identified new interactions with three essential DNA replication enzymes, namely minichromosome 6 (MCM6), DNA polymerase delta (POLD1) and DNA primase (PRIM1). First, we show that in Kit+ hematopoietic cells (TF-1), all components of the pre-replication complex co-immunoprecipitate with LMO2 but not with SCL, suggesting a novel SCL-independent function. Second, LMO2 is recruited to DNA replication origins in these cells together with MCM5. Third, tethering LMO2 to synthetic DNA sequences is sufficient to transform these into origins of replication. Indeed, we show by DNA capture that LMO2 fused to the DNA binding domain of GAL4 is sufficient to recruit DNA replication proteins to GAL4 binding sites on DNA. In vivo, this recruitment is sufficient to drive DNA replication in a manner which is dependent on the integrity of the GAL4 binding sites. These results provide unambiguous evidence for a role of LMO2 in directly controlling DNA replication. Cell cycle and cell differentiation are tightly coordinated during normal hematopoiesis, both during erythroid differentiation and during thymocyte development. We next addressed the functional importance of LMO2 in these two lineages. Erythroid cell differentiation proceeds through different stages from the CD71+Ter119- to the CD71-Ter119+. These stages are also distinguishable by morphological criteria. We observe that LMO2 protein levels directly correlate with the proportion of cells in S phase, i.e. both LMO2 levels and the proportions of cycling cells decrease with terminal erythroid differentiation. Strikingly, lowering LMO2 levels in fetal liver erythroid progenitors via shRNAs decreases the proportion of cells in S phase and arrests Epo-dependent cell growth. Despite a drastic decrease in the numbers of erythroid precursors, these cells differentiate readily to the CD71-Ter119+ stage. Therefore, LMO2 levels dictate cell fate in the erythroid lineage, by favoring DNA replication at the expense of terminal maturation. Conversely, ectopic expression in thymocytes induces DNA replication and drives cells into cell cycle, causing differentiation blockade. Our results define a novel role for the oncogenic transcription factor LMO2 in directly promoting DNA synthesis. To our knowledge, this is the first evidence for a non-transcriptional function of the LMO2 oncogene that drives cell cycle at the expense of differentiation, favouring progenitor cell expansion in the thymus, and causing T-ALL when ectopically expressed in the T lineage. We propose that the non-transcriptional control of DNA replication uncovered here for LMO2 may be a more common function of oncogenic transcription factors than previously appreciated. Disclosures No relevant conflicts of interest to declare.

Methylation of histone H3 lysine 4 (H3K4me) is an evolutionarily conserved modification whose role in the regulation of gene expression has been extensively studied. In contrast, the function of H3K4 acetylation (H3K4ac) has received little attention because of a lack of tools to separate its function from that of H3K4me. Here we show that, in addition to being methylated, H3K4 is also acetylated in budding yeast. Genetic studies reveal that the histone acetyltransferases (HATs) Gcn5 and Rtt109 contribute to H3K4 acetylation in vivo. Whilst removal of H3K4ac from euchromatin mainly requires the histone deacetylase (HDAC) Hst1, Sir2 is needed for H3K4 deacetylation in heterochomatin. Using genome-wide chromatin immunoprecipitation (ChIP), we show that H3K4ac is enriched at promoters of actively transcribed genes and located just upstream of H3K4 tri-methylation (H3K4me3), a pattern that has been conserved in human cells. We find that the Set1-containing complex (COMPASS), which promotes H3K4me2 and -me3, also serves to limit the abundance of H3K4ac at gene promoters. In addition, we identify a group of genes that have high levels of H3K4ac in their promoters and are inadequately expressed in H3-K4R, but not in set1Δ mutant strains, suggesting that H3K4ac plays a positive role in transcription. Our results reveal a novel regulatory feature of promoter-proximal chromatin, involving mutually exclusive histone modifications of the same histone residue (H3K4ac and H3K4me)., Author Summary In the nucleus of mammals and yeast, DNA is packaged by forming complexes with histone proteins in a structure called the nucleosome, the basic building block of chromatin. The tails of the histones protrude from the nucleosome and can be marked on many amino acid residues by chemical modifications such as methylation and acetylation. A highly studied modification, which is robustly associated with active gene promoters, is histone H3 lysine 4 methylation. We describe here a novel modification, histone H3 lysine 4 acetylation (H3K4ac), which can occur on the same lysine of the histone H3 tail (but not at the same time as methylation). We have identified the enzymes responsible for depositing and removing this mark and mapped its distribution throughout the yeast genome. We found that H3K4ac is present on active genes and is important for the full expression of a subset of them. Strikingly, H3K4 methylation was found in the same promoters as H3K4ac and contributes to regulate the abundance and localisation of H3K4ac. This example of cross-talk between two different modifications of the same residue has fundamental implications for understanding how genes are activated and how their packaging in the nucleus controls this process.

BackgroundDot1L, a histone methyltransferase that targets histone H3 lysine 79 (H3K79), has been implicated in gene regulation and the DNA damage response although its functions in these processes remain poorly defined.Methodology/principal findingsUsing the chicken DT40 model system, we generated cells in which the Dot1L gene is disrupted to examine the function and focal recruitment of the 53Bp1 DNA damage response protein. Detailed kinetic and dose response assays demonstrate that, despite the absence of H3K79 methylation demonstrated by mass spectrometry, 53Bp1 focal recruitment is not compromised in these cells. We also describe, for the first time, the phenotypes of a cell line lacking both Dot1L and 53Bp1. Dot1L⁻/⁻ and wild type cells are equally resistant to ionising radiation, whereas 53Bp1⁻/⁻/Dot1L⁻/⁻ cells display a striking DNA damage resistance phenotype. Dot1L and 53Bp1 also affect the expression of many genes. Loss of Dot1L activity dramatically alters the mRNA levels of over 1200 genes involved in diverse biological functions. These results, combined with the previously reported list of differentially expressed genes in mouse ES cells knocked down for Dot1L, demonstrates surprising cell type and species conservation of Dot1L-dependent gene expression. In 53Bp1⁻/⁻ cells, over 300 genes, many with functions in immune responses and apoptosis, were differentially expressed. To date, this is the first global analysis of gene expression in a 53Bp1-deficient cell line.Conclusions/significanceTaken together, our results uncover a negative role for Dot1L and H3K79 methylation in the DNA damage response in the absence of 53Bp1. They also enlighten the roles of Dot1L and 53Bp1 in gene expression and the control of DNA double-strand repair pathways in the context of chromatin.

Candida albicans is a major fungal pathogen that causes serious systemic and mucosal infections in immunocompromised individuals. In yeast, histone H3 Lys56 acetylation (H3K56ac) is an abundant modification regulated by enzymes that have fungal-specific properties, making them appealing targets for antifungal therapy. Here we demonstrate that H3K56ac in C. albicans is regulated by the RTT109 and HST3 genes, which respectively encode the H3K56 acetyltransferase (Rtt109p) and deacetylase (Hst3p). We show that reduced levels of H3K56ac sensitize C. albicans to genotoxic and antifungal agents. Inhibition of Hst3p activity by conditional gene repression or nicotinamide treatment results in a loss of cell viability associated with abnormal filamentous growth, histone degradation and gross aberrations in DNA staining. We show that genetic or pharmacological alterations in H3K56ac levels reduce virulence in a mouse model of C. albicans infection. Our results demonstrate that modulation of H3K56ac is a unique strategy for treatment of C. albicans and, possibly, other fungal infections. -¬ 2010 Nature America, Inc. All rights reserved

Sulfation and phosphorylation are post-translational modifications imparting an isobaric 80-Da addition on the side chain of serine, threonine, or tyrosine residues. These two post-translational modifications are often difficult to distinguish because of their similar MS fragmentation patterns. Targeted MS identification of these modifications in specific proteins commonly relies on their prior separation using gel electrophoresis and silver staining. In the present investigation, we report a potential pitfall in the interpretation of these modifications from silver-stained gels due to artifactual sulfation of serine, threonine, and tyrosine residues by sodium thiosulfate, a commonly used reagent that catalyzes the formation of metallic silver deposits onto proteins. Detailed MS analyses of gel-separated protein standards and Escherichia coli cell extracts indicated that several serine, threonine, and tyrosine residues were sulfated using silver staining protocols but not following Coomassie Blue staining. Sodium thiosulfate was identified as the reagent leading to this unexpected side reaction, and the degree of sulfation was correlated with increasing concentrations of thiosulfate up to 0.02%, which is typically used for silver staining. The significance of this artifact is discussed in the broader context of sulfation and phosphorylation site identification from in vivo and in vitro experiments.

Rtt109, also known as KAT11, is a recently characterized fungal-specific histone acetyltransferase (HAT) that modifies histone H3 lysine 56 (H3K56) to promote genome stability. Rtt109 does not show sequence conservation with other known HATs and depends on association with either of two histone chaperones, Asf1 or Vps75, for HAT activity. Here we report the X-ray crystal structure of an Rtt109-acetyl coenzyme A complex and carry out structure-based mutagenesis, combined with in vitro biochemical studies of the Rtt109-Vps75 complex and studies of Rtt109 function in vivo. The Rtt109 structure reveals noteworthy homology to the metazoan p300/CBP HAT domain but exhibits functional divergence, including atypical catalytic properties and mode of cofactor regulation. The structure reveals a buried autoacetylated lysine residue that we show is also acetylated in the Rtt109 protein purified from yeast cells. Implications for understanding histone substrate and chaperone binding by Rtt109 are discussed.

In eukaryotes, the packaging of DNA into chromatin is essential for cell viability. Several important DNA metabolic events require the transient disruption of chromatin structure, but cells have evolved a number of elaborate pathways that operate throughout the cell cycle to prevent the deleterious effects of chromatin erosion. In this review, we describe a number of distinct nucleosome assembly pathways that function during DNA replication, transcription, cellular senescence and early embryogenesis. In addition, we illustrate some of the physiological consequences associated with defects in nucleosome assembly pathways.

The preparation of quantities of poly(ADP-ribose) glycohydrolase sufficient for detailed structural and enzymatic characterizations has been difficult due to the very low tissue content of the enzyme and its lability in late stages of purification. To date, the only purification of this enzyme to apparent homogeneity has involved a procedure requiring 6 column chromatographic steps. Described here is the preparation of an affinity matrix which consists of ADP-ribose polymers bound to dihydroxyboronyl sepharose. An application is described for the purification of poly(ADP-ribose) glycohydrolase from calf thymus in which a single rapid affinity step was used to replace 3 column chromatographic steps yielding enzyme of greater than 90% purity with a 3 fold increase in yield. This matrix should also prove useful for other studies of ADP-ribose polymer metabolism and related clinical conditions.

Inheritance and maintenance of the DNA sequence and its organization into chromatin are central for eukaryotic life. To orchestrate DNA-replication and -repair processes in the context of chromatin is a challenge, both in terms of accessibility and maintenance of chromatin organization. To meet the challenge of maintenance, cells have evolved efficient nucleosome-assembly pathways and chromatin-maturation mechanisms that reproduce chromatin organization in the wake of DNA replication and repair. The aim of this Review is to describe how these pathways operate and to highlight how the epigenetic landscape may be stably maintained even in the face of dramatic changes in chromatin structure.

Rad53 and Mec1 are protein kinases required for DNA replication and recovery from DNA damage in Saccharomyces cerevisiae. Here, we show that rad53, but not mec1 mutants, are extremely sensitive to histone overexpression, as Rad53 is required for degradation of excess histones. Consequently, excess histones accumulate in rad53 mutants, resulting in slow growth, DNA damage sensitivity, and chromosome loss phenotypes that are significantly suppressed by a reduction in histone gene dosage. Rad53 monitors excess histones by associating with them in a dynamic complex that is modulated by its kinase activity. Our results argue that Rad53 contributes to genome stability independently of Mec1 by preventing the damaging effects of excess histones both during normal cell cycle progression and in response to DNA damage.

The enzyme responsible for maintenance methylation of CpG dinucleotides in vertebrates is DNMT1. The presence of DNMT1 in DNA replication foci raises the issue of whether this enzyme needs to gain access to nascent DNA before its packaging into nucleosomes, which occurs very rapidly behind the replication fork. Using nucleosomes positioned along the 5 S rRNA gene, we find that DNMT1 is able to methylate a number of CpG sites even when the DNA major groove is oriented toward the histone surface. However, we also find that the ability of DNMT1 to methylate nucleosomal sites is highly dependent on the nature of the DNA substrate. Although nucleosomes containing the Air promoter are refractory to methylation irrespective of target cytosine location, nucleosomes reconstituted onto the H19 imprinting control region are more accessible. These results argue that although DNMT1 is intrinsically capable of methylating some DNA sequences even after their packaging into nucleosomes, this is not the case for at least a fraction of DNA sequences whose function is regulated by DNA methylation.

In this issue of Molecular Cell, Ye et al. provide a biological rationale for rapid histone deposition behind the replication fork. They show that defects in nucleosome assembly lead to DNA double-strand breaks and S phase arrest. Their results have important implications for the maintenance of genome integrity in proliferating cells.

An in vitro reconstitution system for the analysis of replication-coupled nucleosome assembly is described. In this “two-step system,” nucleosome assembly is performed in a separate reaction from DNA replication, wherein purified newly replicated DNA remains noncovalently marked for subsequent chromatin assembly factor-1 (CAF-1)-dependent nucleosome assembly. Because the nucleosome assembly is performed separately from the DNA replication step, this system is more versatile and biochemically tractable when compared with nucleosome assembly during simian virus 40 (SV40) DNA replication. The N-terminal domains of histones H3 and H4 play an important but redundant function in nucleosome assembly in the budding yeast, Saccharomyces cerevisiae . It had been proposed that at least one tail of histone H3 or H4 is required for replication-coupled nucleosome assembly. However, we demonstrate that the N-terminal domains of both histone H3 and H4 are dispensable for CAF-1-mediated formation of nucleosome cores onto newly replicated DNA in vitro . CAF-1 and each of its individual subunits stably bound to recombinant (H3.H4) 2 tetramers lacking the N-terminal domains of both H3 and H4. Therefore, the N-terminal tails of histone H3 and H4 that contain the specific acetylation sites are not necessary for CAF-1-dependent nucleosome assembly onto replicated DNA. We suggest that the histone acetylation may be required for a CAF-1 independent pathway or function after deposition, by marking of newly replicated chromatin.