The NH2-terminal php domain of the α subunit of the Escherichia coli replicase binds the ε proofreading subunit

The alpha subunit of the replicase of all bacteria contains a php domain, initially identified by its similarity to histidinol phosphatase but of otherwise unknown function (Aravind, L., and Koonin, E. V. ( 1998) Nucleic Acids Res. 26, 3746 - 3752). Deletion of 60 residues from the NH2 terminus of the alpha php domain destroys epsilon binding. The minimal 255-residue php domain, estimated by sequence alignment with homolog YcdX, is insufficient for is an element of binding. However, a 320-residue segment including sequences that immediately precede the polymerase domain binds is an element of with the same affinity as the 1160-residue full-length alpha subunit. A subset of mutations of a conserved acidic residue (Asp(43) in Escherichia coli alpha) present in the php domain of all bacterial replicases resulted in defects in is an element of binding. Using sequence alignments, we show that the prototypical Gram(+) Pol C, which contains the polymerase and proofreading activities within the same polypeptide chain, has an is an element of-like sequence inserted in a surface loop near the center of the homologous YcdX protein. These findings suggest that the php domain serves as a platform to enable coordination of proofreading and polymerase activities during chromosomal replication.