Mycobacterium tuberculosis sigL encodes an extracytoplasmic function (ECF) sigma factor and is adjacent to a gene for a membrane protein (Rv0736) that contains a conserved HXXXCXXC sequence. This motif is found in anti-sigma factors that regulate several ECF sigma factors, including those that control oxidative stress responses. In this work, SigL and Rv0736 were found to be cotranscribed, and the intracellular domain of Rv0736 was shown to interact specifically with SigL, suggesting that Rv0736 may encode an anti-sigma factor of SigL. An M. tuberculosis sigL mutant was not more susceptible than the parental strain to several oxidative and nitrosative stresses, and sigL expression was not increased in response to these stresses. In vivo, sigL is expressed from a weak SigL-independent promoter and also from a second SigL-dependent promoter. To identify SigL-regulated genes, sigL was overexpressed and microarray analysis of global transcription was performed. Four small operons, sigL (Rv0735)-Rv0736, mpt53 (Rv2878c)-Rv2877c, pks10 (Rv1660)-pks7 (Rv1661), and Rv1139c-Rv1138c, were among the most highly upregulated genes in the sigL-overexpressing strain. SigL-dependent transcription start sites of these operons were mapped, and the consensus promoter sequences TGAACC in the ؊35 region and CGTgtc in the ؊10 region were identified. In vitro, purified SigL specifically initiated transcription from the promoters of sigL, mpt53, and pks10. Additional genes, including four PE_PGRS genes, appear to be regulated indirectly by SigL. In an in vivo murine infection model, the sigL mutant strain showed marked attenuation, indicating that the sigL regulon is important in M. tuberculosis pathogenesis.The Mycobacterium tuberculosis genome encodes 13 sigma factors, of which 10 fall into the extracytoplasmic function (ECF) subfamily. Three of these sigma factor genes are located 5Ј of genes that encode proteins containing an HXXXCXXC motif, which is found in several anti-sigma factors, including Streptomyces coelicolor RsrA (2,15,17,25), Rhodobacter sphaeroides ChrR (23), and M. tuberculosis RshA (35). For both S. coelicolor SigR-RsrA and M. tuberculosis SigH-RshA, it has been demonstrated that this motif in the anti-sigma protein is a key element of a redox switch that affects the interaction between sigma and anti-sigma and thus regulates sigma factor activity in response to intracellular oxidative stress (24, 35). In the case of RsrA and ChrR, this cysteine-rich motif has been shown to bind zinc, and it has been suggested that this is a general property of this motif, leading to the term zinc-associated anti-sigma factor (ZAS) (17,23,24).In addition to sigH, the other M. tuberculosis sigma factor genes linked to these putative ZAS protein genes are sigE and sigL. SigE, like SigH, has been shown to be a key regulator of the mycobacterial response to oxidative and heat stresses (21, 37). In addition, M. tuberculosis SigB, though it is not linked to a similar putative anti-sigma factor gene, is regulated by SigE and SigH and has be...

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Bidirectional replication of the linear chromosomes and plasmids of Streptomyces spp. results in single-strand overhangs at their 3 ends, which contain extensive complex palindromic sequences. The overhangs are believed to be patched by DNA synthesis primed by a terminal protein that remains covalently bound to the 5 ends of the telomeres. We discovered that in vitro a conserved 167-bp telomere DNA binds strongly to RNA polymerase holoenzyme and exhibits promoter activities stronger than those of an rRNA operon. In vivo, the telomere DNA exhibited promoter activity in both orientations on a circular plasmid in Streptomyces. The telomere promoter is also active on a linear plasmid during exponential growth. Such promoter activity in a telomere has not hitherto been observed in eukaryotic or prokaryotic replicons. Streptomyces telomere promoters may be involved in priming the terminal Okazaki fragment (during replication) replicative transfer (during conjugation), or expression of downstream genes (including a conserved ttrA helicase-like gene involved in conjugal transfer). Interestingly, the Streptomyces telomeres also function as a promoter in Escherichia coli and as a transcription enhancer in yeast.The linear chromosomes (33) and plasmids (38) of Streptomyces spp. are capped by terminal proteins (TPs) covalently bound to the 5Ј ends of the DNA. Such TP-capped telomeres are also found in a variety of linear plasmid and viral DNAs of both eukaryotic and prokaryotic origins, including the wellstudied adenoviruses and Bacillus 29 phage, which are replicated from end to end using the TP as a primer for initiation (reviewed in reference 39). In contrast, the linear replicons of Streptomyces spp. are replicated bidirectionally from an internal origin. This mode of replication leaves single-stranded gaps of 250 to 300 nucleotides (nt) at the 3Ј ends (16; C.-H. Huang, unpublished results), which are supposedly patched by TPprimed DNA synthesis (reviewed in reference 15). For essentially all linear chromosomes and most linear plasmids of Streptomyces spp., the telomere sequences are highly conserved for about 200 bp and contain extensive palindromic sequences capable of forming complex secondary structures (27). These archetypal telomeres are capped by TP encoded by tpg (4, 45). Several Tpg proteins have been identified (4, 45), and they are highly conserved in size (184 to 185 amino acids) and sequences.Atypical telomere sequences have been found in a few linear plasmids (32, 47) and one chromosome (23) of Streptomyces spp. For example, the telomeres of linear plasmid SCP1 of Streptomyces coelicolor differ significantly from the archetypal telomeres in primary (32) and secondary (7) structures. They are capped by a 259-amino-acid TP (encoded by the tpc gene on SCP1), which shares no homology with Tpgs.Little is known about the TP-primed end patching during replication of the Streptomyces linear replicons. An in vitro deoxynucleotidylation study using a crude extract of Streptomyces identified a Thr residue on Tpg (of ...