Insights into the Specificity of Lysine Acetyltransferases

Reversible lysine acetylation by protein acetyltransferases is a conserved regulatory mechanism that controls diverse cellular pathways. Gcn5-related N-acetyltransferases (GNATs), named after their founding member, are found in all domains of life. GNATs are known for their role as histone acetyltransferases, but non-histone bacterial protein acetytransferases have been identified. Only structures of GNAT complexes with short histone peptide substrates are available in databases. Given the biological importance of this modification and the abundance of lysine in polypeptides, how specificity is attained for larger protein substrates is central to understanding acetyl-lysine-regulated networks. Here we report the structure of a GNAT in complex with a globular protein substrate solved to 1.9 Å. GNAT binds the protein substrate with extensive surface interactions distinct from those reported for GNAT-peptide complexes. Our data reveal determinants needed for the recognition of a protein substrate and provide insight into the specificity of GNATs.     

A Single-Molecule Surface-Based Platform to Detect the Assembly and Function of the Human RNA Polymerase II Transcription Machinery

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The ethylmalonyl-CoA pathway is used in place of the glyoxylate cycle by Methylobacterium extorquens AM1 during growth on acetate

Acetyl-CoA assimilation was extensively studied in organisms harboring the glyoxylate cycle. In this study, we analyzed the metabolism of the facultative methylotroph Methylobacterium extorquens AM1, which lacks isocitrate lyase, the key enzyme in the glyoxylate cycle, during growth on acetate. MS/MS-based proteomic analysis revealed that the protein repertoire of M. extorquens AM1 grown on acetate is similar to that of cells grown on methanol and includes enzymes of the ethylmalonyl-CoA (EMC) pathway that were recently shown to operate during growth on methanol. Dynamic 13C labeling experiments indicate the presence of distinct entry points for acetate: the EMC pathway and the TCA cycle. 13C steady-state metabolic flux analysis showed that oxidation of acetyl-CoA occurs predominantly via the TCA cycle and that assimilation occurs via the EMC pathway. Furthermore, acetyl-CoA condenses with the EMC pathway product glyoxylate, resulting in malate formation. The latter, also formed by the TCA cycle, is converted to phosphoglycerate by a reaction sequence that is reversed with respect to the serine cycle. Thus, the results obtained in this study reveal the utilization of common pathways during the growth of M. extorquens AM1 on C1 and C2 compounds, but with a major redirection of flux within the central metabolism. Furthermore, our results indicate that the metabolic flux distribution is highly complex in this model methylotroph during growth on acetate and is fundamentally different from organisms using the glyoxylate cycle.     

RNA聚合酶II中的CTD结构在mRNA转录和加工偶联过程中的重要作用

RNA聚合酶II最大亚基末端的羧基端结构域（CTD）是由以七氨基酸（-Tyr-Ser-Pro-Thr-Ser- Pro-Ser-）为重复单位的高度保守的重复序列组成的。因其结构与功能上的特殊性,它在近年来受到学术界的广泛关注。诸多的实验表明,该结构可通过与mRNA加工因子的相互作用而对其加工过程产生直接或间接的影响,在mRNA的转录和加工的偶联过程中具有重要的作用。本文即是根据近年来的实验结果对其结构和功能做出的综述性介绍。     

Initiation by a Eukaryotic RNA-dependent RNA Polymerase Requires Looping of the Template End and Is Influenced by the Template-tailing Activity of an Associated Uridyltransferase

A conserved family of eukaryotic RNA-dependent RNA polymerases (RDRs) initiates or amplifies the production of small RNAs to provide sequence specificity for gene regulation by Argonaute/Piwi proteins. RDR-dependent silencing processes affect the genotype-phenotype relationship in many eukaryotes, but the principles that underlie the specificity of RDR template selection and product synthesis are largely unknown. Here, we characterize the initiation specificity of the Tetrahymena RDR, Rdr1, as a heterologously expressed single subunit and in the context of its biologically assembled multisubunit complexes (RDRCs). Truncation analysis of recombinant Rdr1 revealed domain requirements different from those of the only other similarly characterized RDR, suggesting that there are subfamilies of the RDR enzyme with distinct structural requirements for activity. We demonstrate an apparently obligate Rdr1 mechanism of initiation in which the template end is looped to provide the hydroxyl group priming the synthesis of dsRNA. RDRC subunits with poly(U) polymerase activity can act on the template end prior to looping to increase the duplex length of product, thus impacting the small RNA sequences generated by the RDRC-coupled Dicer. Overall, our findings give new perspective on mechanisms of RDR initiation and demonstrate that non-RDR subunits of an RDRC can affect the specificity of product synthesis.     

Nitrogen Oxyanion-dependent Dissociation of a Two-component Complex That Regulates Bacterial Nitrate Assimilation

Nitrogen is an essential nutrient for growth and is readily available to microbes in many environments in the form of ammonium and nitrate. Both ions are of environmental significance due to sustained use of inorganic fertilizers on agricultural soils. Diverse species of bacteria that have an assimilatory nitrate/nitrite reductase system (NAS) can use nitrate or nitrite as the sole nitrogen source for growth when ammonium is limited. In Paracoccus denitrificans, the pathway-specific two-component regulator for NAS expression is encoded by the nasT and nasS genes. Here, we show that the putative RNA-binding protein NasT is a positive regulator essential for expression of the nas gene cluster (i.e. nasABGHC). By contrast, a nitrogen oxyanion-binding sensor (NasS) is required for nitrate/nitrite-responsive control of nas gene expression. The NasS and NasT proteins co-purify as a stable heterotetrameric regulatory complex, NasS-NasT. This protein-protein interaction is sensitive to nitrate and nitrite, which cause dissociation of the NasS-NasT complex into monomeric NasS and an oligomeric form of NasT. NasT has been shown to bind the leader RNA for nasA. Thus, upon liberation from the complex, the positive regulator NasT is free to up-regulate nas gene expression.     

Regulation of Response Regulator Autophosphorylation through Interdomain Contacts

DNA-binding response regulators (RRs) of the OmpR/PhoB subfamily alternate between inactive and active conformational states, with the latter having enhanced DNA-binding affinity. Phosphorylation of an aspartate residue in the receiver domain, usually via phosphotransfer from a cognate histidine kinase, stabilizes the active conformation. Many of the available structures of inactive OmpR/PhoB family proteins exhibit extensive interfaces between the N-terminal receiver and C-terminal DNA-binding domains. These interfaces invariably involve the α4-β5-α5 face of the receiver domain, the locus of the largest differences between inactive and active conformations and the surface that mediates dimerization of receiver domains in the active state. Structures of receiver domain dimers of DrrB, DrrD, and MtrA have been determined, and phosphorylation kinetics were analyzed. Analysis of phosphotransfer from small molecule phosphodonors has revealed large differences in autophosphorylation rates among OmpR/PhoB RRs. RRs with substantial domain interfaces exhibit slow rates of phosphorylation. Rates are greatly increased in isolated receiver domain constructs. Such differences are not observed between autophosphorylation rates of full-length and isolated receiver domains of a RR that lacks interdomain interfaces, and they are not observed in histidine kinase-mediated phosphotransfer. These findings suggest that domain interfaces restrict receiver domain conformational dynamics, stabilizing an inactive conformation that is catalytically incompetent for phosphotransfer from small molecule phosphodonors. Inhibition of phosphotransfer by domain interfaces provides an explanation for the observation that some RRs cannot be phosphorylated by small molecule phosphodonors in vitro and provides a potential mechanism for insulating some RRs from small molecule-mediated phosphorylation in vivo.