Studies of SpoIIAB mutant proteins elucidate the mechanisms that regulate the developmental transcription factor σ^F in Bacillus subtilis

σ^F, the first compartment-specific sigma factor of sporulation, is regulated by an anti-sigma factor, SpoIIAB (AB) and its antagonist SpoIIAA (AA). AB can bind to σ^F in the presence of ATP or to AA in the presence of ADP; in addition, AB can phosphorylate AA. The ability of AB to switch between its two binding partners regulates σ^F. Early in sporulation, AA activates σ^F by releasing it from its complex with AB. We have previously proposed a reaction scheme for the phosphorylation of AA by AB which accounts for AA's regulatory role. A crucial feature of this scheme is a conformational change in AB that accompanies its switch in binding partner. In the present study, we have studied three AB mutants, all of which have amino-acid replacements in the nucleotide-binding region; AB-E104K (Glu¹⁰⁴ → Lys) and AB-T49K (Thr⁴⁹ → Lys) fail to activate σ^F, and AB-R105A (Arg¹⁰⁵ → Ala) activates it prematurely. We used techniques of enzymology, surface plasmon resonance and fluorescence spectroscopy to analyse the defects in each mutant. AB-E104K was deficient in binding to AA, AB-T49K was deficient in binding to ADP and AB-R105A bound ADP exceptionally strongly. Although the release of σ^F from all three mutant proteins was impaired, and all three failed to undergo the wild-type conformational change when switching binding partners, the phenotypes of the mutant cells were best accounted for by the properties of the respective AB species in forming complexes with AA and ADP. The behaviour of the mutants enables us to propose convincing mechanisms for the regulation of σ^F in wild-type bacteria.