Temporal and spatial control of the mother-cell regulatory gene spoIIID of Bacillus subtilis.

Gene expression during endospore formation in Bacillus subtilis is compartmentalized between the mother-cell and forespore chambers of the sporangium, which follow separate pathways of cellular differentiation. The earliest acting regulatory gene so far identified in the mother-cell line of gene expression is spoIIID, whose product is required for the transcription of the composite gene (sigK) encoding the mother-cell RNA polymerase sigma-factor sigma K and for the chromosomal rearrangement that gives rise to the composite gene. Here we report the nucleotide sequence of spoIIID and studies on the temporal, spatial, and genetic control of its expression during sporulation. We show that the deduced spoIIID gene product, a 93-residue-long polypeptide, is a previously identified transcription factor that is known to activate the promoter for the sigK gene in vitro. Expression of spoIIID is largely confined to the mother-cell chamber of the sporangium and is turned on at, or shortly before, the time (hour 3 of sporulation) that the mother-cell chromosome is rearranged and transcription of the sigK gene commences. This gene expression depends strongly on the sporulation sigma-factor sigma E and partially on the spoIIID gene product, itself. We conclude that the timing and compartmentalization of the rearrangement and transcription of the sigK gene and, hence, of subsequent gene activation in the mother cell, are, in part, direct consequences of the temporal and spatial control of spoIIID gene expression.