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-l ike sigma factors of RNA polymerase. While transcription of the sigA gene i s growth rate independent, sigB transcription is increased during entry int o stationary phase. The sigil gene transcription is unresponsive to environ mental stress but that of sigB is very responsive, more so in stationary-ph ase growth than in log-phase cultures. These data suggest that SigA is a pr imary sigma factor which, like sigma(70), controls the transcription of the housekeeping type of promoters. In contrast, SigB, although showing some o verlap in function with SigA, is more like the alternative sigma factor, si gma(S), 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 sigma(70) promoter. The half-life of the sigA trans cript was very long, and this is likely to play an important part in its re gulation. In contrast, the half-life of the sigB transcript was short, abou t 2 min. These results demonstrate that the sigB gene may control the regul ons of stationary phase and general stress resistance, while sigA may be in volved in the housekeeping regulons.