Posttranslational modification of the umuD-encoded subunit of Escherichia coli DNA polymerase V regulates its interactions with the β processivity clamp

The Escherichia coli umuDC (pol V) gene products participate in both a DNA damage checkpoint control and translesion DNA synthesis. Interactions of the two umuD gene products, the 139-aa UmuD and the 115-aa UmuD′ proteins, with components of the replicative DNA polymerase (pol III), are important for determining which biological role the umuDC gene products will play. Here we report our biochemical characterizations of the interactions of UmuD and UmuD′ with the pol III β processivity clamp. These analyses demonstrate that UmuD possesses a higher affinity for β than does UmuD′ because of the N-terminal arm of UmuD (residues 1-39), much of which is missing in UmuD′. Furthermore, we have identified specific amino acid residues of UmuD that crosslink to β with p-azidoiodoacetanilide, defining the domain of UmuD important for the interaction. We have recently proposed a model for the solution structure of UmuD₂ in which the N-terminal arm of each protomer makes extensive contacts with the C-terminal globular domain of its intradimer partner, masking part of each surface. Taken together, our findings suggest that UmuD₂ has a higher affinity for the β-clamp than does UmuD′₂ because of the structures of its N-terminal arms. Viewed in this way, posttranslational modification of UmuD, which entails the removal of its N-terminal 24 residues to yield UmuD′, acts in part to attenuate the affinity of the umuD gene product for the β-clamp. Implications of these structure-function analyses for the checkpoint and translesion DNA synthesis functions of the umuDC gene products are discussed.