Control of σ factor activity during Bacillus subtilis sporulation

Abstract

When starved, Bacillus subtilis undergoes asymmetric division to produce two cell types with different fates. The larger mother cell engulfs the smaller forespore, then nurtures it and, eventually, lyses to release a dormant, environmentally resistant spore. Driving these changes is a programme of transcriptional gene regulation. At the heart of the programme are σ factors, which become active at different times, some only in one cell type or the other, and each directing RNA polymerase to transcribe a different set of genes. The activity of each σ factor in the cascade is carefully regulated by multiple mechanisms. In some cases, novel proteins control both σ factor activity and morphogenesis, co-ordinating the programme of gene expression with morphological change. These bifunctional proteins, as well as other proteins involved in a factor activation, and even precursors of σ factors themselves, are targeted to critical locations, allowing the mother cell and forespore to communicate with each other and to co-ordinate their programmes of gene expression. This signalling can result in proteolytic σ factor activation. Other mechanisms, such as an anti-σ factor and, perhaps, proteolytic degradation, prevent σ factors from becoming active in the wrong cell type. Accessory transcription factors modulate RNA polymerase activity at specific promoters. Negative feedback loops limit σ factor production and facilitate the transition from one σ factor to the next. Together, the mechanisms controlling σ factor activity ensure that genes are expressed at the proper time and level in each cell type.