Three sites of contact between the Bacillus subtilis transcription factor σ(F) and its antisigma factor SpoIIAB

Abstract

The developmental regulatory protein σ(F) of Bacillus subtilis, a member of the σ70-family of RNA polymerase sigma factors, is regulated negatively by the antisigma factor SpoIIAB, which binds to σ(F) to form an inactive complex. Complex formation between SpoIIAB, which contains an inferred adenosine nucleotide binding pocket, and σ(F) is stimulated strongly by the presence of ATP. Here we report that SpoIIAB contacts σ(F) at three widely spaced binding surfaces corresponding to conserved regions 2.1, 3.1, and 4.1 of σ70-like sigma factors. This conclusion is based on binding studies between SpoIIAB and truncated portions of σ(F) the isolation of mutants of σ(F) 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 σ(F) 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 σ(F). 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.