Temporal expression of the Bacillus subtilis secA gene, encoding a centralcomponent of the preprotein translocase

In Bacillus subtilis, the secretion of extracellular proteins strongly incr eases upon transition from exponential growth to the stationary growth phas e.It is not known whether the amounts of some or all components of the prot ein translocation apparatus are concomitantly increased in relation to the increased export activity. In this study, we analyzed the transcriptional o rganization and temporal expression of the secA gene, encoding a central co mponent of the B. subtilis preprotein translocase, We found that secA and t he downstream gene (prfB) constitute an operon that is transcribed from a v egetative (sigma(A)-dependent) promoter located upstream of secA. Furthermo re, using different independent methods, we found that secA expression occu rred mainly in the exponential growth phase, reaching a maximal value almos t precisely at the transition from exponential growth to the stationary gro wth phase. Following to this maximum, the de novo transcription of secA sha rply decreased to a low basal level. Since at the time of maximal secA tran scription the secretion activity of B. subtilis strongly increases, our res ults clearly demonstrate that the expression of at least one of the central components of the B, subtilis protein export apparatus is adapted to the i ncreased demand for protein secretion. Possible mechanistic consequences ar e discussed.