The specificity of DNA-dependent RNA polymerase for target promoters is largely due to the replaceable σ subunit that it carries. Multiple σ proteins, each conferring a unique promoter preference on RNA polymerase, are likely to be present in all bacteria; however, their abundance and diversity have been best characterized in Bacillus subtilis, the bacterium in which multiple σ factors were first discovered. The 10 sigma factors thus far identified in B. subtilis directly contribute to the bacterium's ability to control gene expression. These proteins are not merely necessary for the expression of those operons whose promoters they recognize; in many instances, their appearance within the cell is sufficient to activate these operons. This review describes the discovery of each of the known B. subtilis sigma factors, their characteristics, the regulons they direct, and the complex restrictions placed on their synthesis and activities. These controls include the anticipated transcriptional regulation that modulates the expression of the sigma factor structural genes but, in the case of several of the B. subtilis sigma factors, go beyond this, adding novel posttranslational restraints on σ factor activity. Two of the sigma factors (σ(E) and σ(K)) are, for example, synthesized as inactive precursor proteins. Their activities are kept in check by 'pro-protein' sequences which are cleaved from the precursor molecules in response to intercellular cues. Other sigma factors (σ(B), σ(F), and σ(G)) are inhibited by 'anti-σ factor' proteins that sequester them into complexes which block their ability to form RNA polymerase holoenzymes. The anti-sigma factors are, in turn, opposed by additional proteins which participate in the sigma factors' release. The devices used to control σ factor activity in B. subtilis may prove to be as widespread as multiple σ factors themselves, providing ways of coupling σ factor activation to environmental or physiological signals that cannot be readily joined to other regulatory mechanisms.
ASJC Scopus subject areas
- Applied Microbiology and Biotechnology