Properties of the phosphorylation reaction catalyzed by SpoIIAB that help to regulate sporulation of Bacillus subtilis

Phosphorylation of SpoIIAA on Ser-58 catalyzed by SpoIIAB is important in the regulation of sporulation of Bacillus subtilis. Nucleotide binding experiments showed that the affinity of SpoIIAB for ATP was greatly increased in the presence of SpoIIAA or a mutant SpoIIAA in which Ser-58 had been changed to alanine. Study of the phosphorylation reaction showed that the K-m for ATP and the K-i for ADP were both about 1 mu M. The kinetics of phosphorylation of SpoIIAA by SpoIIAB were biphasic, comprising a rapid phase (leading to phosphorylation of 1 mol of SpoIIAA/mol of SpoIIAB) followed by a slower, steady-state phase. In the steady state, the rate-determining step proved to be the dissociation of a SpoIIAB-ADP complex. The rate of this dissociation was not affected significantly by changes in the concentration of ATP.