Fate of the SpoIIAB*-ADP liberated after SpoIIAB phosphorylates SpoIIAA ofBacillus subtilis

Phosphorylation of SpoIIAA catalyzed by SpoIIAB helps to regulate the first sporulation-specific sigma factor, sigma(F), of Bacillus subtilis. The ste ady-state rate of phosphorylation is known to be exceptionally slow and to be limited by the return of the protein kinase, SpoIIAB, to a catalytically active state. Previous work from this laboratory has suggested that, after catalyzing the phosphorylation, SpoIIAB is in a form (SpoIIAB*) that does not readily release ADP. We now show that the rate of release of ADP from t he SpoIIAB*-ADP complex was much diminished by the presence of unreacted Sp oIIAA, suggesting that SpoIIAA can form a long-lived ternary complex with S poIIAB*-ADP in which the SpoIIAB* form is stabilized. In kinetic studies of the phosphorylation of SpoIIAA, the ternary complex SpoIIAA-SpoIIAB*-ADP c ould be distinguished from the short-lived complex SpoIIAA-SpoIIAB-ADP, whi ch can be readily produced in the absence of an enzymatic reaction.