Dissection of the Functional and Structural Domains of Phosphorelay Histidine Kinase A of Bacillus subtilis

The initiation of sporulation in Bacillus subtilis results primarily from phosphoryl group input into the phosphorelay by histidine kinases, the major kinase being kinase A. Kinase A is active as a homodimer, the protomer of which consists of an approximately 400-amino-acid N-terminal putative signal-sensing region and a 200-amino-acid C-terminal autokinase. On the basis of sequence similarity, the N-terminal region may be subdivided into three PAS domains: A, B, and C, located from the N- to the C-terminal end. Proteolysis experiments and two-hybrid analyses indicated that dimerization of the N-terminal region is accomplished through the PAS-B/PAS-C region of the molecule, whereas the most amino-proximal PAS-A domain is not dimerized. N-terminal deletions generated with maltose binding fusion proteins showed that an intact PAS-A domain is very important for enzymatic activity. Amino acid substitution mutations in PAS-A as well as PAS-C affected the in vivo activity of kinase A, suggesting that both PAS domains are required for signal sensing. The C-terminal autokinase, when produced without the N-terminal region, was a dimer, probably because of the dimerization required for formation of the four-helix-bundle phosphotransferase domain. The truncated autokinase was virtually inactive in autophosphorylation with ATP, whereas phosphorylation of the histidine of the phosphotransfer domain by back reactions from Spo0F~P appeared normal. The phosphorylated autokinase lost the ability to transfer its phosphoryl group to ADP, however. The N-terminal region appears to be essential both for signal sensing and for maintaining the correct conformation of the autokinase component domains.