Possible intermediate steps in the evolution of a prokaryotic developmental system.
Sigma factors (sigma) are transcription factors that operate global switches in gene expression in prokaryotes. They work by directing core RNA polymerase to specific cis-acting promoter sequences; each sigma has a cognate class of promoters with specific sequence characteristics. In Bacillus subtilis four different sigma factors have been implicated in the regulation of gene expression during spore formation, which is a simple differentiation system involving two cell types. In this review I show how the modern developmental system may have arisen from a primitive organism that used only two sigma factors, by a series of steps involving gene duplication and divergence. The increasing sophistication of eukaryotic developmental systems may reflect similar evolutionary processes.