Sporulation operon spoIVF and the characterization of mutations that uncouple mother-cell from forespore gene expression in Bacillus subtilis.

During the process of endospore formation in Bacillus subtilis the appearance of the mother-cell transcription factor sigma K by conversion from its inactive precursor pro-sigma K is coupled to events under the control of the forespore transcription factor sigma G. This intercompartmental coupling is believed to be mediated by the products of a sporulation locus called spoI V F because certain bypass-of-forespore (bof) mutations that map at the spoI V F locus relieve the dependence of pro-sigma K processing on the action of sigma G in the forespore. We now report that spoI V F is a two-cistron operon whose transcription is under the control of the sporulation transcription factor sigma E and whose products are likely to be integral membrane proteins. We show that the products of both the promoter-proximal (spoI V F A) and promoter-distal (spoI V F B) cistrons are required for spore formation at 37 degrees C, but that the spoI V F A gene product is dispensable at 30 degrees C. The bypass-of-forespore mutations are located at the extreme 3' end of the spoI V F A cistron, one such mutation causing a proline to serine substitution eight residues from the COOH terminus of SpoIVFA and another (a nonsense mutation) causing the absence of the terminal six amino acid residues of the protein. We also show that at a permissive temperature for spore formation spoI V F A null mutants exhibit a bypass-of-forespore phenotype. We hypothesize that SpoIVFA functions positively in stabilizing SpoIVFB, which we propose is thermolabile in the absence of the promoter-proximal gene product, and negatively in inhibiting the action of SpoIVFB. A model for intercompartmental coupling is presented in which SpoIVFB promotes pro-sigma K processing in response to a signal from the forespore that relieves or otherwise counteracts the inhibitory effect of SpoIVFA on SpoIVFB.