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The olfactory system translates a vast array of volatile chemicals into diverse odor perceptions and innate behaviors. Odor detection in the mouse nose is mediated by 1,000 different odorant receptors (ORs) and 14 trace amine-associated receptors (TAARs). ORs are used in a combinatorial manner to encode the unique identities of myriad odorants. However, some TAARs appear to be linked to innate responses, raising questions about regulatory mechanisms that might segregate OR and TAAR expression in appropriate subsets of olfactory sensory neurons (OSNs). Here, we report that OSNs that express TAARs comprise at least two subsets that are biased to express TAARs rather than ORs. The two subsets are further biased in Taar gene choice and their distribution within the sensory epithelium, with each subset preferentially expressing a subgroup of Taar genes within a particular spatial domain in the epithelium. Our studies reveal one mechanism that may regulate the segregation of Olfr (OR) and Taar expression in different OSNs: the sequestration of Olfr and Taar genes in different nuclear compartments. Although most Olfr genes colocalize near large central heterochromatin aggregates in the OSN nucleus, Taar genes are located primarily at the nuclear periphery, coincident with a thin rim of heterochromatin. Taar-expressing OSNs show a shift of one Taar allele away from the nuclear periphery. Furthermore, examination of hemizygous mice with a single Taar allele suggests that the activation of a Taar gene is accompanied by an escape from the peripheral repressive heterochromatin environment to a more permissive interior chromatin environment.

The human mixed-lineage leukemia 5 (MLL5) protein mediates hematopoietic cell homeostasis, cell cycle, and survival; however, the molecular basis underlying MLL5 activities remains unknown. Here, we show that MLL5 is recruited to gene-rich euchromatic regions via the interaction of its plant homeodomain finger with the histone mark H3K4me3. The 1.48-Å resolution crystal structure of MLL5 plant homeodomain in complex with the H3K4me3 peptide reveals a noncanonical binding mechanism, whereby K4me3 is recognized through a single aromatic residue and an aspartate. The binding induces a unique His-Asp swapping rearrangement mediated by a C-terminal α-helix. Phosphorylation of H3T3 and H3T6 abrogates the association with H3K4me3 in vitro and in vivo, releasing MLL5 from chromatin in mitosis. This regulatory switch is conserved in the Drosophila ortholog of MLL5, UpSET, and suggests the developmental control for targeting of H3K4me3. Together, our findings provide first insights into the molecular basis for the recruitment, exclusion, and regulation of MLL5 at chromatin.

A state-dependent dynamic network is a collection of elements that interact through a network, whose geometry evolves as the state of the elements changes over time. The genome is an intriguing example of a state-dependent network, where chromosomal geometry directly relates to genomic activity, which in turn strongly correlates with geometry. Here we examine various aspects of a genomic state-dependent dynamic network. In particular, we elaborate on one of the important ramifications of viewing genomic networks as being state-dependent, namely, their controllability during processes of genomic reorganization such as in cell differentiation.

Although the importance of chromosome organization during mitosis is clear, it remains to be determined whether the nucleus assumes other functionally relevant chromosomal topologies. We have previously shown that homologous chromosomes have a tendency to associate during hematopoiesis according to their distribution of coregulated genes, suggesting cell-specific nuclear organization. Here, using the mathematical approaches of distance matrices and coupled oscillators, we model the dynamic relationship between gene expression and chromosomal associations during the differentiation of a multipotential hematopoietic progenitor. Our analysis reveals dramatic changes in total genomic order: Commitment of the progenitor results in an initial increase in entropy at both the level of gene coregulation and chromosomal organization, which we suggest represents a phase transition, followed by a progressive decline in entropy during differentiation. The stabilization of a highly ordered state in the differentiated cell types results in lineage-specific chromosomal topologies and is related to the emergence of coherence-or self-organization-between chromosomal associations and coordinate gene regulation. We discuss how these observations may be generally relevant to cell fate decisions encountered by progenitor/stem cells.

Silencing of multiple cancer-related genes is associated with de novo methylation of linked CpG islands. Additionally, bivalent histone modification profiles characterized by the juxtaposition of active and inactive histone marks have been observed in genes that become hypermethylated in cancer. It is unknown how these ambiguous epigenetic states are maintained and how they interrelate with adjacent genomic regions with different epigenetic landscapes. Here, we present the analysis of a set of neighboring genes, including many frequently silenced in colon cancer cells, in a chromosomal region at 5q35.2 spanning 1.25 Mb. Promoter DNA methylation occurs only at genes maintained at a low transcriptional state and is characterized by the presence of bivalent histone marks, namely trimethylation of lysines 4 and 27 in histone 3. Chemically induced hyperacetylation and DNA demethylation lead to up-regulation of silenced genes in this locus yet do not resolve bivalent domains into a domain-wide active chromatin conformation. In contrast, active genes in the region become down-regulated after drug treatment, accompanied by a partial loss of chromatin domain boundaries and spreading of the inactive histone mark trimethylated lysine 27 in histone 3. Our results demonstrate that bivalent domains mark the promoters of genes that will become DNA methylated in adult tumor cells to enforce transcriptional silence. These bivalent domains not only remain upon drug induced gene reactivation, but also spread over adjacent CpG islands. These results may have important implications in understanding and managing epigenetic therapies of cancer.

Duplication of the genome during the S phase of the cell cycle does not occur simultaneously; rather, different sequences are replicated at different times. The replication timing of specific sequences can change during development; however, the determinants of this dynamic process are poorly understood. To gain insights into the contribution of developmental state, genomic sequence, and transcriptional activity to replication timing, we investigated the timing of DNA replication at high resolution along an entire human chromosome (chromosome 22) in two different cell types. The pattern of replication timing was correlated with respect to annotated genes, gene expression, novel transcribed regions of unknown function, sequence composition, and cytological features. We observed that chromosome 22 contains regions of early- and late-replicating domains of 100 kb to 2 Mb, many (but not all) of which are associated with previously described chromosomal bands. In both cell types, expressed sequences are replicated earlier than nontranscribed regions. However, several highly transcribed regions replicate late. Overall, the DNA replication-timing profiles of the two different cell types are remarkably similar, with only nine regions of difference observed. In one case, this difference reflects the differential expression of an annotated gene that resides in this region. Novel transcribed regions with low coding potential exhibit a strong propensity for early DNA replication. Although the cellular function of such transcripts is poorly understood, our results suggest that their activity is linked to the replication-timing program.

Small Maf proteins serve as dual-function transcription factors through an exchange of their heterodimerization partners. For example, as heterodimers with hematopoietic cell-specific p45 NF-E2 or NF-E2-related factors (Nrf), they activate the beta-globin or antioxidative stress enzyme heme oxygenase 1 (HO-1) genes, respectively. In contrast, together with Bach1, they repress these same genes. However, the signals leading to this partner exchange are not known. Using chromatin immunoprecipitation assays in NIH 3T3 cells, we show that heme, an inducer of ho-1, promotes displacement of Bach1 from the MafK-occupied ho-1 enhancers, which is followed by Nrf2 binding to these elements. Whereas histone H3 at the ho-1 enhancers and promoter is hyperacetylated irrespective of gene activity, exposure of cells to heme results in de novo hyperacetylation and hypermethylation of histone H3 in the transcribed region. These data indicate that, under normal conditions, the chromatin structure of ho-1 is in a preactivation state, but transcription is repressed by Bach1. Heme induces switching of Maf dimers, resulting in ho-1 expression. Heme also promotes displacement of Bach1 from the beta-globin locus control region without affecting MafK binding in murine erythroleukemia cells. Thus, heme functions as a signaling molecule for gene expression in higher eukaryotes.

In eukaryotic genomes, transcriptionally active regions are interspersed with silent chromatin that may repress genes in its vicinity. Chromatin insulators are elements that can shield a locus from repressive effects of flanking chromatin. Few such elements have been characterized in higher eukaryotes, but transcriptional activating elements are an invariant feature of active loci and have been shown to suppress transgene silencing. Hence, we have assessed the ability of a transcriptional activator to cause chromatin insulation, i.e., to relieve position effects at transgene integration sites in cultured cells. The transgene contained a series of binding sites for the metal-inducible transcriptional activator MTF, linked to a GFP reporter. Clones carrying single integrated transgenes were derived without selection for expression, and in most clones the transgene was silent. Induction of MTF resulted in transition of the transgene from the silent to the active state, prolongation of the active state, and a marked narrowing of the range of expression levels at different genomic sites. At one genomic site, prolonged induction of MTF resulted in suppression of transgene silencing that persisted after withdrawal of the induction stimulus. These results are consistent with MTF acting as a chromatin insulator and imply that transcriptional activating elements can insulate active loci against chromatin repression.

Murine erythroleukemia (MEL) cells are a model system to study reorganization of the eukaryotic nucleus during terminal differentiation. Upon chemical induction, MEL cells undergo erythroid differentiation, leading to activation of globin gene expression. Both processes strongly depend on the transcriptional activator NF-E2. Before induction of differentiation, both subunits of the NF-E2 heterodimer are present, but little DNA-binding activity is detectable. Using immunofluorescence microscopy, we show that the two NF-E2 subunits occupy distinct nuclear compartments in uninduced MEL cells; the smaller subunit NF-E2p18 is found primarily in the centromeric heterochromatin compartment, whereas the larger subunit NF-E2p45 occupies the euchromatin compartment. Concomitant with the commitment period of differentiation that precedes globin gene activation, NF-E2p18, along with other transcriptional repressors, relocates to the euchromatin compartment. Thus, relocation of NF-E2 p18 may be a rate-limiting step in formation of an active NF-E2 complex. To understand the mechanisms of NF-E2 localization, we show that centromeric targeting of NF-E2p18 requires dimerization, but not with an erythroid-specific partner, and that the transactivation domain of NF-E2p45 may be necessary and sufficient to prevent its localization in centromeric heterochromatin. Finally, using fluorescence in situ hybridization, we show that, upon differentiation, the beta-globin gene loci relocate away from heterochromatin compartments to euchromatin. This relocation correlates with both transcriptional activation of the globin locus and relocation of NF-E2p18 away from heterochromatin, suggesting that these processes are linked.

Locus control regions (LCRs) are defined by their ability to confer high-level tissue-specific expression to linked genes in transgenic assays. Previously, we reported that, at its native site, the murine beta-globin LCR is required for high-level beta-globin gene expression, but is not required to initiate an open chromatin conformation of the locus. To further investigate the mechanism of LCR-mediated transcriptional enhancement, we have analyzed allele-specific beta-globin expression and the pattern of histone acetylation in the presence and absence of the LCR. In single cells from mice heterozygous for a deletion of the LCR, beta-globin expression from the LCR-deleted allele is consistently low ( approximately 1-4% of wild type). Thus, the endogenous LCR enhances globin gene expression by increasing the rate of transcription from each linked allele rather than by increasing the probability of establishing transcription per se. Furthermore, in erythroid cells from mice homozygous for the highly expressing wild-type beta-globin locus, hyperacetylation of histones H3 and H4 is localized to the LCR and active genes. In mice homozygous for the LCR deletion reduced histone hyperacetylation is observed in LCR proximal sequences; however, deletion of the LCR has no effect on the localized hyperacetylation of the genes. Together, our results suggest that, in its native genomic context, the LCR follows the rate model of enhancer function, and that the developmentally specific hyperacetylation of the globin genes is independent of both the rate of transcription and the presence of the LCR.

The mouse beta-globin gene locus control region (LCR), located upstream of the beta-globin gene cluster, is essential for the activated transcription of genes in the cluster. The LCR contains multiple binding sites for transactivators, including Maf-recognition elements (MAREs). However, little is known about the specific proteins that bind to these sites or the time at which they bind during erythroid differentiation. We have performed chromatin immunoprecipitation experiments to determine the recruitment of the erythroid-specific transactivator p45 NF-E2/MafK (p18 NF-E2) heterodimer and small Maf proteins to various regions in the globin gene locus before and after the induction of murine erythroleukemia (MEL) cell differentiation. We report that, before induction, the LCR is occupied by small Maf proteins, and, on erythroid maturation, the NF-E2 complex is recruited to the LCR and the active globin promoters, even though the promoters do not contain MAREs. This differentiation-coupled recruitment of NF-E2 complex correlates with a greater than 100-fold increase in beta-major globin transcription, but is not associated with a significant change in locus-wide histone H3 acetylation. These findings suggest that the beta-globin gene locus exists in a constitutively open chromatin conformation before terminal differentiation, and we speculate that recruitment of NF-E2 complex to the LCR and active promoters may be a rate-limiting step in the activation of beta-globin gene expression.

By sequencing regions flanking the beta-globin gene complex in mouse (Hbbc) and human (HBBC), we have shown that the beta-globin gene cluster is surrounded by a larger cluster of olfactory receptor genes (ORGs). To facilitate sequence comparisons and to investigate the regulation of ORG expression, we have mapped 5' sequences of mRNA from olfactory epithelium encoding beta-globin-proximal ORGs. We have found that several of these genes contain multiple noncoding exons that can be alternatively spliced. Surprisingly, the only common motifs found in the promoters of these genes are a "TATA" box and a purine-rich motif. Sequence comparisons between human and mouse reveal that most of the conserved regions are confined to the coding regions and transcription units of the genes themselves, but a few blocks of conserved sequence also are found outside of ORG transcription units. The possible influence of beta-globin regulatory sequences on ORG expression in olfactory epithelium was tested in mice containing a deletion of the endogenous beta-globin locus control region, but no change in expression of the neighboring ORGs was detected. We evaluate the implications of these results for possible mechanisms of regulation of ORG transcription.

In mouse and human, the beta-globin genes reside in a linear array that is associated with a positive regulatory element located 5' to the genes known as the locus control region (LCR). The sequences of the mouse and human beta-globin LCRs are homologous, indicating conservation of an essential function in beta-globin gene regulation. We have sequenced regions flanking the beta-globin locus in both mouse and human and found that homology associated with the LCR is more extensive than previously known, making up a conserved block of approximately 40 kb. In addition, we have identified DNaseI-hypersensitive sites within the newly sequenced regions in both mouse and human, and these structural features also are conserved. Finally, we have found that both mouse and human beta-globin loci are embedded within an array of odorant receptor genes that are expressed in olfactory epithelium, and we also identify an olfactory receptor gene located 3' of the beta-globin locus in chicken. The data demonstrate an evolutionarily conserved genomic organization for the beta-globin locus and suggest a possible role for the beta-globin LCR in control of expression of these odorant receptor genes and/or the presence of mechanisms to separate regulatory signals in different tissues.

Enhancers are defined by their ability to stimulate gene activity from remote sites and their requirement for promoter-proximal upstream activators to activate transcription. Here we demonstrate that recruitment of the p300/CBP-associated factor PCAF to a reporter gene is sufficient to stimulate promoter activity. The PCAF-mediated stimulation of transcription from either a distant or promoter-proximal position depends on the presence of an upstream activator (Sp1). These data suggest that acetyltransferase activity may be a primary component of enhancer function, and that recruitment of polymerase and enhancement of transcription are separable. Transcriptional activation by PCAF requires both its acetyltransferase activity and an additional activity within its N terminus. We also show that the simian virus 40 enhancer and PCAF itself are sufficient to counteract Mad-mediated repression. These results are compatible with recent models in which gene activity is regulated by the competition between deacetylase-mediated repression and enhancer-mediated recruitment of acetyltransferases.

The mouse metallothionein-I (mMT-I) promoter is activated by the metal response element-binding transcription factor (MTF), which binds metal response elements (MREs) when stimulated with heavy metals. We analyzed eight K562 erythroleukemia cell clones, each carrying a single integrated copy of an mMT-I/beta-geo construct, using a system that can independently assess the level of beta-geo expression and the rate at which it is silenced. In these clones, basal expression and rate of silencing vary widely and independently with integration site. This implies that the rates of transcription and of silencing are separate properties determined by interaction of the regulatory elements of the transgene with the site of integration. Induction of the mMT-I promoter with zinc both increases expression level and strongly retards silencing of beta-geo expression. At a given integration site, expression level and silencing are affected coordinately by induction. Taken together with earlier studies of distant metal-responsive elements, these results suggest that distance from the promoter may determine whether a factor can increase transcription rate. Stimulation of an MRE can both increase transcription and overcome repressive effects of chromatin; we suggest that these functions are linked.