SUMMARY Histone methylation occurs on both lysine and arginine residues, and its dynamic regulation plays a critical role in chromatin biology. Here we identify the UHRF1 PHD finger (PHDUHRF1), an important regulator of DNA CpG methylation, as a histone H3 unmodified arginine 2 (H3R2) recognition modality. This conclusion is based on binding studies and cocrystal structures of PHDUHRF1 bound to histone H3 peptides, where the guanidinium group of unmodified R2 forms an extensive intermolecular hydrogen bond network, with methylation of H3R2, but not H3K4 or H3K9, disrupting complex formation. We have identified direct target genes of UHRF1 from microarray and ChIP studies. Importantly, we show that UHRF1's ability to repress its direct target gene expression is dependent on PHDUHRF1 binding to unmodified H3R2, thereby demonstrating the functional importance of this recognition event and supporting the potential for crosstalk between histone arginine methylation and UHRF1 function.

The recent discovery of fatty acyl-AMP ligases (FAALs) in Mycobacterium tuberculosis (Mtb) provided a new perspective to fatty acid activation dogma. These proteins convert fatty acids to corresponding adenylates, which is an intermediate of acyl-CoA-synthesizing fatty acyl-CoA ligases (FACLs). Presently, it is not evident how obligate pathogens like Mtb have evolved such new themes of functional versatility and whether the activation of fatty acids to acyl-adenylates could indeed be a general mechanism. Here, based on elucidation of the first structure of a FAAL protein and by generating loss- as well as gain-of-function mutants that interconvert FAAL and FACL activities, we demonstrate that an insertion motif dictates formation of acyl-adenylate. Since FAALs in Mtb are crucial nodes in biosynthetic network of virulent lipids, inhibitors directed against these proteins provide a unique multi-pronged approach of simultaneously disrupting several pathways.

Cytosine DNA methylation is evolutionarily ancient, and in eukaryotes this epigenetic modification is associated with gene silencing. Proteins with SRA (SET-or RING-associated) methyl-binding domains are required for the establishment and/or maintenance of DNA methylation in both plants and mammals. The 5-methyl-cytosine (5mC)-binding specificity of several SRA domains have been characterized, and each one has a preference for DNA methylation in different sequence contexts. Here we demonstrate through mobility shift assays and calorimetric measurements that the SU(VAR)3-9 HOMOLOG 5 (SUVH5) SRA domain differs from other SRA domains in that it can bind methylated DNA in all contexts to similar extents. Crystal structures of the SUVH5 SRA domain bound to 5mC-containing DNA in either the fully or hemimethylated CG context or the methylated CHH context revealed a dual flip-out mechanism where both the 5mC and a base (5mC, C, or G, respectively) from the partner strand are simultaneously extruded from the DNA duplex and positioned within binding pockets of individual SRA domains. Our structure-based in vivo studies suggest that a functional SUVH5 SRA domain is required for both DNA methylation and accumulation of the H3K9 dimethyl modification in vivo, suggesting a role for the SRA domain in recruitment of SUVH5 to genomic loci.

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The low-molecular-weight protein tyrosine phosphatase (LMWPTPase) belongs to a distinctive class of phosphotyrosine phosphatases widely distributed among prokaryotes and eukaryotes. We report here the crystal structure of LMWPTPase of microbial origin, the first of its kind from Mycobacterium tuberculosis. The structure was determined to be two crystal forms at 1.9-and 2.5-Å resolutions. These structural forms are compared with those of the LMWPTPases of eukaryotes. Though the overall structure resembles that of the eukaryotic LMWPTPases, there are significant changes around the active site and the protein tyrosine phosphatase (PTP) loop. The variable loop forming the wall of the crevice leading to the active site is conformationally unchanged from that of mammalian LMWPTPase; however, differences are observed in the residues involved, suggesting that they have a role in influencing different substrate specificities. The single amino acid substitution (Leu12Thr [underlined below]) in the consensus sequence of the PTP loop, CTGNI CRS, has a major role in the stabilization of the PTP loop, unlike what occurs in mammalian LMWPTPases. A chloride ion and a glycerol molecule were modeled in the active site where the chloride ion interacts in a manner similar to that of phosphate with the main chain nitrogens of the PTP loop. This structural study, in addition to identifying specific mycobacterial features, may also form the basis for exploring the mechanism of the substrate specificities of bacterial LMWPTPases.