3 SITES OF CONTACT BETWEEN THE BACILLUS-SUBTILIS TRANSCRIPTION FACTORSIGMA(F) AND ITS ANTISIGMA FACTOR SPOIIAB

The developmental regulatory protein sigma(F) of Bacillus subtilis, a member of the sigma(70)-family of RNA polymerase sigma factors, is reg ulated negatively by the antisigma factor SpoIIAB, which binds to sigm a(F) to form an inactive complex. Complex formation between SpoIIAB, w hich contains an inferred adenosine nucleotide binding pocket, and sig ma(F) is stimulated strongly by the presence of ATP. Here we report th at SpoIIAB contacts sigma(F) at three widely spaced binding surfaces c orresponding to conserved regions 2.1, 3.1, and 4.1 of sigma(70)-like sigma factors. This conclusion is based on binding studies between Spo IIAB and truncated portions of sigma(F), the isolation of mutants of s igma(F) that were partially resistant to inhibition by SpoIIAB in vivo and were defective in binding to the antisigma factor in vitro, and t he creation of alanine substitution mutants of regions 2.1, 3.1, or 4. 1 of sigma(F) that were impaired in complex formation. Because the int eraction 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 sigma(F). Finally, we discuss the poss ibility that another antisigma factor, unrelated to SpoIIAB, may inter act 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.