Regulation of Prespore-Specific Transcription during Sporulation in Bacillus subtilis

Abstract

During spore formation in the Gram positive bacterium, Bacillus subtilis, asymmetric cell division produces a small prespore cell and a much larger mother cell. The two cells then collaborate in an intricate developmental process which culminates with lysis of the mother cell and release of the mature spore (Fig. 1). Many genes involved in sporulation are known and the regulatory pathways controlling their expression are well understood (Errington, 1993; Stragier and Losick, 1996). The main changes in gene expression during sporulation are controlled by four sigma factors, each of which directs RNA polymerise to recognise new promoter sequences and thus turn on new sets of genes. Two of the sporulation-specific sigma factors act successively in the prespore, σ^F and then σ^G: the others act successively in the mother cell; σ^E followed by σ^K (Fig. 1). All four sigma factors are tightly regulated, at both transcriptional and post-translational levels. The complex regulation serves to ensure that the sigma factors only become active at the proper time and place in the developmental process. Although the different regulatory programmes of the prespore and mother cell operate in separate compartments, they are by no means independent. Indeed, in genetic experiments the four sigma factor activities behave as if they operate in a linear dependent sequence, thus:

$${\sigma ^F} \to {\sigma ^E} \to {\sigma ^G} \to {\sigma ^K}$$

This hierarchy indicates that the mechanisms linking the synthesis or activation of each successive sigma factor involve intercellular signal transduction pathways. The nature of these pathways are not yet understood in detail but will be of considerable interest.