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The mechanism by which the recently identified DNA modification 5-formylcytosine ( f C) is recognized by epigenetic writer and reader proteins is not known. Recently, an unusual DNA structure, F-DNA, has been proposed as the basis for enzyme recognition of clusters of f C. We used NMR and X-ray crystallography to compare several modified DNA duplexes with unmodified analogs and found that in the crystal state the duplexes all belong to the A family, whereas in solution they are all members of the B family. We found that, contrary to previous findings, f C does not significantly affect the structure of DNA, although there are modest local differences at the modification sites. Hence, global conformation changes are unlikely to account for the recognition of this modified base, and our structural data favor a mechanism that operates at base-pair resolution for the recognition of f C by epigenome-modifying enzymes.

MetR, a LysR-type transcriptional regulator (LTTR), has been extensively studied owing to its role in the control of methionine biosynthesis in proteobacteria. A MetR homodimer binds to a 24-base-pair operator region of the met genes and specifically recognizes the interrupted palindromic sequence 5'-TGAA-N5-TTCA-3'. Mechanistic details underlying the interaction of MetR with its target DNA at the molecular level remain unknown. In this work, the crystal structure of the DNA-binding domain (DBD) of MetR was determined at 2.16 Å resolution. MetR-DBD adopts a winged-helix-turn-helix (wHTH) motif and shares significant fold similarity with the DBD of the LTTR protein BenM. Furthermore, a data-driven macromolecular-docking strategy was used to model the structure of MetR-DBD bound to DNA, which revealed that a bent conformation of DNA is required for the recognition helix α3 and the wing loop of the wHTH motif to interact with the major and minor grooves, respectively. Comparison of the MetR-DBD-DNA complex with the crystal structures of other LTTR-DBD-DNA complexes revealed residues that may confer operator-sequence binding specificity for MetR. Taken together, the results show that MetR-DBD uses a combination of direct base-specific interactions and indirect shape recognition of the promoter to regulate the transcription of met genes.

Archaeal viruses have evolved to infect hosts often thriving in extreme conditions such as high temperatures. However, there is a paucity of information on archaeal virion structures, genome packaging, and determinants of temperature resistance. The rod-shaped virus APBV1 (Aeropyrum pernix bacilliform virus 1) is among the most thermostable viruses known; it infects a hyperthermophile Aeropyrum pernix, which grows optimally at 90 °C. Here we report the structure of APBV1, determined by cryo-electron microscopy at near-atomic resolution. Tight packing of the major virion glycoprotein (VP1) is ensured by extended hydrophobic interfaces, and likely contributes to the extreme thermostability of the helical capsid. The double-stranded DNA is tightly packed in the capsid as a left-handed superhelix and held in place by the interactions with positively charged residues of VP1. The assembly is closed by specific capping structures at either end, which we propose to play a role in DNA packing and delivery. [ABSTRACT FROM AUTHOR]

In Bacillus subtilis the concentration ofl-arginine is controlled by the transcriptional regulator AhrC, which interacts with 18 bp DNA operator sites called ARG boxes in the promoters of arginine biosynthetic and catabolic operons. AhrC is a 100 kDa homohexamer, with each subunit having two domains. The C-terminal domains form the core, mediating intersubunit interactions and binding of the co-repressorl-arginine, whilst the N-terminal domains contain a winged helix–turn–helix DNA-binding motif and are arranged around the periphery. The N-terminal domain of AhrC has been expressed, purified and characterized and it has been shown that the fragment still binds DNA operators as a recombinant monomer. The DNA-binding domain has also been crystallized and the crystal structure refined to 1.0 Å resolution is presented. [ABSTRACT FROM AUTHOR]

The four-way (Holliday) DNA junction is the central intermediate in homologous recombination, a ubiquitous process that is important in DNA repair and generation of genetic diversity. The penultimate stage of recombination requires resolution of the DNA junction into nicked-duplex species by the action of a junction-resolving enzyme, examples of which have been identified in a wide variety of organisms. These enzymes are nucleases that are highly selective for the structure of branched DNA. The mechanism of this selectivity has, however, been unclear in the absence of structural data. Here we present the crystal structure of the junction-resolving enzyme phage T7 endonuclease I in complex with a synthetic four-way DNA junction. Although the enzyme is structure-selective, significant induced fit occurs in the interaction, with changes in the structure of both the protein and the junction. The dimeric enzyme presents two binding channels that contact the backbones of the junction’s helical arms over seven nucleotides. These interactions effectively measure the relative orientations and positions of the arms of the junction, thereby ensuring that binding is selective for branched DNA that can achieve this geometry. [ABSTRACT FROM AUTHOR]

The response regulator VicR from the Gram-positive bacterium Enterococcus faecalis forms part of the two-component signal transduction system of the YycFG subfamily. The structure of the DNA-binding domain of VicR, VicRc, has been solved and belongs to the winged helix–turn–helix family. It is very similar to the DNA-binding domains of Escherichia coli PhoB and OmpR, despite low sequence similarity, but differs in two important loops. The α-loop, which links the two helices of the helix–turn–helix motif, is similar to that of PhoB, where it has been implicated in contacting the σ subunit of RNA polymerase, but differs from that of OmpR. Conversely, the loop following the helix–turn–helix motif is similar to that of OmpR and differs from that of PhoB. YycF/VicR, PhoB and Bacillus subtilis PhoP regulators all recognize almost identical DNA sequences and although there is currently no experimental evidence linking this loop with the DNA, the structure is consistent with possible involvement in selective DNA recognition or binding. [ABSTRACT FROM AUTHOR]

The junction-resolving enzyme endonuclease I is selective for the structure of the DNA four-way (Holliday) junction. The enzyme binds to a four-way junction in two possible orientations, with a 4:1 ratio, opening the DNA structure at the centre and changing the global structure into a 900 cross of approximately coaxial helices. The nuclease cleaves the continuous strands of the junction in each orientation. Binding leads to pronounced regions of protection of the DNA against hydroxyl radical attack. Using all this information together with the known structure of the enzyme and the structure of the BglI-DNA complex, we have constructed a model of the complex of endonuclease I and a DNA junction. This shows how the enzyme is selective for the structure of a four-way junction, such that both continuous strands can be accommodated into the two active sites so that a productive resolution event is possible. [ABSTRACT FROM AUTHOR]

The crystal structure of the type II restriction endonuclease BglI bound to DNA containing its specific recognition sequence has been determined at 2.2 Å resolution. This is the first structure of a restriction endonuclease that recognizes and cleaves an interrupted DNA sequence, producing 3′ overhanging ends. BglI is a homodimer that binds its specific DNA sequence with the minor groove facing the protein. Parts of the enzyme reach into both the major and minor grooves to contact the edges of the bases within the recognition half-sites. The arrangement of active site residues is strikingly similar to other restriction endonucleases, but the coordination of two calcium ions at the active site gives new insight into the catalytic mechanism. Surprisingly, the core of a BglI subunit displays a striking similarity to subunits of EcoRVand PvuII, but the dimer structure is dramatically different. The BglI—DNA complex demonstrates, for the first time, that a conserved subunit fold can dimerize in more than one way, resulting in different DNA cleavage patterns. [ABSTRACT FROM AUTHOR]