SpoIIAB is an anti-sigma factor that binds to and inhibits transcription by regulatory protein sigma F from Bacillus subtilis.

The sigma F factor is a regulatory protein that is responsible for directing gene expression in the forespore compartment of developing cells of the spore-forming soil bacterium Bacillus subtilis. The sigma F factor is encoded by the promoter-distal member of sporulation operon spoIIA, which consists of cistrons called spoIIAA, spoIIAB, and spoIIAC. Genetic evidence indicates that the activity of sigma F is negatively regulated by the product (SpoIIAB) of the spoIIAB cistron. We now report that SpoIIAB is capable of binding to sigma F and inhibiting its capacity to direct transcription by core RNA polymerase from the promoter for a forespore-expressed gene. SpoIIAB is an anti-sigma factor that may be directly involved in the compartmentalization of sigma F-directed gene expression.