Forespore-specific transcription of the lonB gene during sporulation in Bacillus subtilis

The Bacillus subtilis genome encodes two members of the Lon family of proka ryotic ATP-dependent proteases. One, LonA, is produced in response to tempe rature, osmotic, and oxidative stress and has also been implicated in preve nting sigma (G) activity under nonsporulation conditions. The second is enc oded by the lonB gene, which resides immediately upstream from lonA. Here w e report that transcription of lonB occurs during sporulation under sigma ( F) control and thus is restricted to the prespore compartment of sporulatin g cells. First, expression of a lonB-lacZ transcriptional fusion was abolis hed in strains unable to produce sigma (F) but remained unaffected upon dis ruption of the genes encoding the early and late mother cell regulators sig ma (E) and sigma (K) or the late forespore regulator oc. Second, the fluore scence of strains harboring a lonB-gfp fusion was confined to the prespore compartment and depended on sigma (F) production. Last, primer extension an alysis of the lonB transcript revealed -10 and -35 sequences resembling the consensus sequence recognized by sigma (F)-containing RNA polymerase. We f urther show that the lonB message accumulated as a single monocistronic tra nscript during sporulation, synthesis of which required sigma (F) activity. Disruption of the lonB gene did not confer any discernible sporulation phe notype to otherwise wild-type cells, nor did expression of lonB from a mult icopy plasmid. In contrast, expression of a fusion of the lonB promoter to the lonA gene severely reduced expression of the sigma (G)-dependent sspE g ene and the frequency of sporulation. In confirmation of earlier observatio ns, we found elevated levels of sigma (F)-dependent activity in a spoIIIE47 mutant, in which the lonB region of the chromosome is not translocated int o the prespore. Expression of either lonB or the P-lonB-lonA fusion from a plasmid in the spoIIIE47 mutant reduced sigma (F)-dependent activity to wil d-type levels. The results suggest that both Lon;S and LonB can prevent abn ormally high sigma (F) activity but that only LonA can negatively regulate sigma (G).