Endospore formation in Bacillus subtilis is a morphologically complex process in which the bacterium divides into two compartments (forespore and mother cell) that follow different developmental paths. Compartment-specific transcription in the forespore is initiated by RNA polymerase containing sigma F, and results in the forespore-specific production of sigma G, which directs most of the subsequent forespore-specific transcription. The activity of sigma F is thought to be restricted to the forespore by the sigma factor antagonist SpoIIAB. We used antibodies against SpoIIAB to monitor its accumulation during sporulation. We found that SpoIIAB accumulates early after the initiation of sporulation, and that it was present in the mother-cell compartment 2h after sigma F became active in the forespore. SpoIIAB disappeared preferentially from the forespore during development, and its disappearance from the forespore compartment correlated with the activation of sigma G in that compartment, raising the possibility that SpoIIAB may be involved regulating sigma G activity. We tested whether SpoIIAB could antagonize sigma G activity by replacing the sigma F-dependent promoter that drives expression of spoIIIG, the structural gene for sigma G, with a sigma H-dependent promoter. This resulted in a lytic phenotype that was suppressed by the simultaneous expression of a plasmid-borne copy of spoIIAB. This suggests that SpoIIAB can suppress this effect of sigma G expression. Moreover, these cells formed spores efficiently.(ABSTRACT TRUNCATED AT 250 WORDS)