EFFECTS OF LYSINE-TO-GLYCINE MUTATIONS IN THE ATP-BINDING CONSENSUS SEQUENCES IN THE ADDA AND ADDB SUBUNITS ON THE BACILLUS-SUBTILIS ADDAB ENZYME-ACTIVITIES

The N-terminal regions of both subunits AddA and AddB of the Bacillus subtilis AddAB enzyme contain amino acid sequences, designated motif I , which are commonly found in ATP-binding enzymes. The functional sign ificance of the motif I regions was studied by replacing the highly co nserved lysine residues of the regions in both subunits by glycines an d by examination of the resulting mutant enzymes with respect to their enzymatic properties. This study shows that the mutation in subunit A ddB hardly affected the ATPase, helicase, and exonuclease activities o f the AddAB enzyme. However, the mutation in subunit AddA drastically reduced these activities, as well as the k(cat) for ATP hydrolysis. Th e apparent K-m for ATP in ATP hydrolysis did not significantly deviate from that of the wild-type enzyme. These results suggest that the lys ine residue in motif I of subunit AddA of the AddAB enzyme is not esse ntial for the binding of the nucleotide but has a role in ATP hydrolys is, which is required for the exonuclease and helicase activities of t he enzyme.