Processing of the mother-cell σ factor, σØK, may depend on events occurring in the forespore during Bacillus subtilis development

During sporulation of the Gram-positive bacterium Bacillus subtilis, transcription of genes encoding spore coat proteins in the mother-cell compartment of the sporangium is controlled by RNA polymerase containing the σ subunit called σ^K. Based on comparison of the N-terminal amino acid sequence of σ^K with the nucleotide sequence of the gene encoding σ^K (sigK), the primary product of sigK was inferred to be a pro-protein (pro-σ^K) with 20 extra amino acids at the N terminus. Using antibodies generated against pro-σ^K, we have detected pro-σ^K beginning at the third hour of sporulation and σ^K beginning about 1 hr later. Even when pro-σ^K is expressed artificially during growth and throughout sporulation, σ^K appears at the normal time and expression of a σ^K-controlled gene occurs normally. These results suggest that pro-σ^K is an inactive precursor that is proteolytically processed to active σ^K in a developmentally regulated fashion. Mutations that block forespore gene expression block accumulation of σ^K but not accumulation of pro-σ^K, suggesting that pro-σ^K processing is a regulatory device that couples the programs of gene expression in the two compartments of the sporangium. We propose that this regulatory device ensures completion of forespore morphogenesis prior to the synthesis in the mother-cell of spore coat proteins that will encase the forespore.