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ThesigA andsigB genes ofMycobacterium tuberculosis encode two sigma 70-like sigma factors of RNA polymerase. While transcription of thesigA gene is growth rate independent,sigB transcription is increased during entry into stationary phase. ThesigA gene transcription is unresponsive to environmental stress but that ofsigB is very responsive, more so in stationary-phase growth than in log-phase cultures. These data suggest that SigA is a primary sigma factor which, like ς70 , controls the transcription of the housekeeping type of promoters. In contrast, SigB, although showing some overlap in function with SigA, is more like the alternative sigma factor, ςS , which controls the transcription of the gearbox type of promoters. Primer extension analysis identified the RNA start sites for both genes as 129 nucleotides upstream to the GTG start codon ofsigA and 27 nucleotides from the ATG start codon ofsigB . The −10 promoter ofsigA but not that ofsigB was similar to the ς70 promoter. The half-life of thesigA transcript was very long, and this is likely to play an important part in its regulation. In contrast, the half-life of thesigB transcript was short, about 2 min. These results demonstrate that thesigB gene may control the regulons of stationary phase and general stress resistance, whilesigA may be involved in the housekeeping regulons.

Transcription of Two Sigma 70 Homologue Genes, sigA and sigB , in Stationary-Phase Mycobacterium tuberculosis