Three sites of contact between the Bacillus subtilis transcription factor sigmaF and its antisigma factor SpoIIAB

The developmental regulatory protein sigmaF of Bacillus subtilis, a member of the sigma70-family of RNA polymerase sigma factors, is regulated negatively by the antisigma factor SpoIIAB, which binds to sigmaF to form an inactive complex. Complex formation between SpoIIAB, which contains an inferred adenosine nucleotide binding pocket, and sigmaF is stimulated strongly by the presence of ATP. Here we report that SpoIIAB contacts sigmaF at three widely spaced binding surfaces corresponding to conserved regions 2.1, 3.1, and 4.1 of sigma70-like sigma factors. This conclusion is based on binding studies between SpoIIAB and truncated portions of sigmaF, the isolation of mutants of sigmaF that were partially resistant to inhibition by SpoIIAB in vivo and were defective in binding to the antisigma factor in vitro, and the creation of alanine substitution mutants of regions 2.1, 3.1, or 4.1 of sigmaF that were impaired in complex formation. Because the interaction of SpoIIAB with all three binding surfaces was stimulated by ATP, we infer that ATP induces a conformational change in SpoIIAB that is needed for tight binding to sigmaF. Finally, we discuss the possibility that another antisigma factor, unrelated to SpoIIAB, may interact with its respective sigma factor in a similar topological pattern of widely spaced binding surfaces located in or near conserved regions 2.1, 3.1, and 4.1.