Synthesis of σ²⁹, an RNA polymerase specificity determinant, is a developmentally regulated event in Bacillus subtilis

Using an immunological probe, we have determined that the synthesis of the Bacillus subtilis RNA polymerase promoter specificity determinant σ²⁹ is a developmentally regulated event. σ²⁹ is absent from vegetatively growing cells but is abundant in sporulating cells for a restricted (2-h) period during differentiation (hour 2 to hour 4 into the sporeforming process). The narrowness of this period suggests that σ²⁹ is a regulatory factor that directs the transcription of a subpopulation of genes at a precise, intermediate stage of spore formation. This view predicts that σ²⁹ should be dispensable for early sporulation events. We verified this prediction by an analysis of σ²⁹ accumulation in mutants that are blocked at different stages of sporulation in which we show that cells can advance to at least an intermediate point in development (stage III) in the absence of detectable σ²⁹. Lastly, our anti-σ²⁹ antibody probe detected a second, previously unrecognized protein in Bacillus cell extracts that may be a precursor to σ²⁹. This protein, P³¹ (molecular weight, 31,000) is synthesized earlier in sporulation than is σ²⁹. It has a peptide profile that is similar to σ²⁹ and is present in all Bacillus subtilis Spo^- mutants that were tested and found to still be able to accumulate σ²⁹.