The C53 and C37 subunits of RNA polymerase III (pol III) form a subassembly that is required for efficient termination; pol III lacking this subcomplex displays increased processivity of RNA chain elongation. We show that the C53/C37 subcomplex additionally plays a role in formation of the initiation-ready open promoter complex similar to that of the Brf1 N-terminal zinc ribbon domain. In the absence of C53 and C37, the transcription bubble fails to stably propagate to and beyond the transcriptional start site even when the DNA template is supercoiled. The C53/C37 subcomplex also stimulates the formation of an artificially assembled elongation complex from its component DNA and RNA strands. Protein-RNA and protein-DNA photochemical cross-linking analysis places a segment of C53 close to the RNA 3 end and transcribed DNA strand at the catalytic center of the pol III elongation complex. We discuss the implications of these findings for the mechanism of transcriptional termination by pol III and propose a structural as well as functional correspondence between the C53/C37 subcomplex and the RNA polymerase II initiation factor TFIIF. RNA polymerase III (pol III)3 transcribes genes encoding short RNA products that are essential for protein synthesis (e.g. tRNAs, 5 S rRNA), RNA processing (e.g. U6 small nuclear RNA, the RNA subunit of RNase P), protein transport (7SL RNA of the signal recognition particle), and diverse other functions, particularly in higher eukaryotes (1). Pol III is brought to the transcriptional start sites of its genes through interactions with its central 3-subunit initiation factor, TFIIIB (subunits Brf1, TBP, and Bdp1). TFIIIB, in turn, is assembled on DNA upstream of the transcriptional start site in a largely sequenceindependent process by its six-subunit assembly factor, TFIIIC, which binds to two promoter elements (boxA and boxB) located downstream of the transcriptional start site. An additional initiation factor, TFIIIA, serves solely as the adaptor for assembling TFIIIC on 5 S rRNA genes. Vertebrates contain an additional form of TFIIIB in which its paralogue Brf2 replaces Brf1 for transcription of a class of genes that use the SNAP c complex in place of TFIIIC as their TFIIIB-assembly factor (2, 3). Once it has been assembled onto DNA, Saccharomyces cerevisiae TFIIIB suffices for recruiting pol III for robust and accurately initiating transcription in vitro (4); at least in yeast, this also appears to be the case in vivo (5). The complexity of pol III contrasts with the apparent simplicity of its core transcription apparatus (the TFIIIB⅐DNA complex and diverse assembly factors). Five pol III subunits have no counterparts in the considerably less complex pol II (with its merely 12 subunits). Three of these pol III subunits (C82, C34, and C31) form a subassembly that interacts with the TFIIIB⅐DNA complex and is required for specifically initiating transcription (6). Two subunits, C53 and C37, form a subassembly that limits the processivity of pol III elongation and contributes to the abil...

Naturally occurring variants of the enzyme chorismate mutase are known to exist that exhibit diversity in enzyme structure, regulatory properties, and association with other proteins.

IdeR (iron-dependent regulator) is a key regulator of virulence factors and iron acquisition systems in Mycobacterium tuberculosis. Despite the wealth of information available on IdeR-regulated genes of M.tuberculosis, there is still an underlying possibility that there are novel genes/pathways that have gone undetected, the identification of which could give new insights into understanding the pathogenesis of M.tuberculosis. We describe an in silico approach employing the positional relative entropy method to identify potential IdeR binding sites in the upstream sequences of all the 3919 ORFs of M.tuberculosis. While many of the predictions made by this approach overlapped with the ones already identified by microarray experiments and binding assays, pointing to the accuracy of our method, a few genes for which there has been no evidence for IdeR regulation were additionally identified. Our results have implications on the iron-dependent regulatory mechanism of M.tuberculosis vis-a-vis the activity of urease operon and novel transcription regulators and transporters.

Prephenate dehydratase (PDT) is a key regulatory enzyme in L-phenylalanine biosynthesis. In Mycobacterium tuberculosis, expression of pheA, the gene encoding PDT, has been earlier reported to be iron-dependent (1, 2). We report that M. tuberculosis pheA is also regulated at the protein level by aromatic amino acids. All of the three aromatic amino acids (phenylalanine, tyrosine, and tryptophan) are potent allosteric activators of M. tuberculosis PDT. We also provide in vitro evidence that M. tuberculosis PDT does not possess any chorismate mutase activity, which suggests that, unlike many other enteric bacteria (where PDT exists as a fusion protein with chorismate mutase), M. tuberculosis PDT is a monofunctional and a non-fusion protein. Finally, the biochemical and biophysical properties of the catalytic and regulatory domains (ACT domain) of M. tuberculosis PDT were studied to observe that, in the absence of the ACT domain, the enzyme not only loses its regulatory activity but also its catalytic activity. These novel results provide evidence for a monofunctional prephenate dehydratase enzyme from a pathogenic bacterium that exhibits extensive allosteric activation by aromatic amino acids and is absolutely dependent upon the presence of catalytic as well as the regulatory domains for optimum enzyme activity.

Chorismate mutase catalyzes the first committed step toward the biosynthesis of the aromatic amino acids, phenylalanine and tyrosine. While this biosynthetic pathway exists exclusively in the cell cytoplasm, the Mycobacterium tuberculosis enzyme has been shown to be secreted into the extracellular medium. The secretory nature of the enzyme and its existence in M. tuberculosis as a duplicated gene are suggestive of its role in host-pathogen interactions. We report here the crystal structure of homodimeric chorismate mutase (Rv1885c) from M. tuberculosis determined at 2.15 A resolution. The structure suggests possible gene duplication within each subunit of the dimer (residues 35-119 and 130-199) and reveals an interesting proline-rich region on the protein surface (residues 119-130), which might act as a recognition site for protein-protein interactions. The structure also offers an explanation for its regulation by small ligands, such as tryptophan, a feature previously unknown in the prototypical Escherichia coli chorismate mutase. The tryptophan ligand is found to be sandwiched between the two monomers in a dimer contacting residues 66-68. The active site in the "gene-duplicated" monomer is occupied by a sulfate ion and is located in the first half of the polypeptide, unlike in the Saccharomyces cerevisiae (yeast) enzyme, where it is located in the later half. We hypothesize that the M. tuberculosis chorismate mutase might have a role to play in host-pathogen interactions, making it an important target for designing inhibitor molecules against the deadly pathogen.