Transcription of two sigma 70 homologue genes, sigA and sigB, in stationary-phase Mycobacterium tuberculosis.

The sigA and sigB genes of Mycobacterium tuberculosis encode two sigma 70-like sigma factors of RNA polymerase. While transcription of the sigA gene is growth rate independent, sigB transcription is increased during entry into stationary phase. The sigA gene transcription is unresponsive to environmental stress but that of sigB 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 sigma70, 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, sigmaS, 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 of sigA and 27 nucleotides from the ATG start codon of sigB. The -10 promoter of sigA but not that of sigB was similar to the sigma70 promoter. The half-life of the sigA transcript was very long, and this is likely to play an important part in its regulation. In contrast, the half-life of the sigB transcript was short, about 2 min. These results demonstrate that the sigB gene may control the regulons of stationary phase and general stress resistance, while sigA may be involved in the housekeeping regulons.