MicroReview Cascades of sigma factors revisited

Programmed gene expression during the process of endospore formation in Bacillus subtilis is governed by the successive appearance of five developmental sigma factors. These sigma factors are encoded by genes in which mutations arrest sporulation at a defined stage. These genes are turned on sequentially and depend for their own transcription on the activity of a previously synthesized sigma factor. Superimposed on the regulation of synthesis of the sigma factors are post-transcriptional control mechanisms that couple the activation of the developmental sigma factors to the course of sporulation. Here we review evidence indicating that these developmental transcription factors comprise a regulatory cascade in the order sigma H----sigma F----sigma E----sigma G----sigma K in which the activity of each sigma factor depends on the action of the preceding sigma factor in the cascade.