Two processivity clamp interactions differentially alter the dual activities of UmuC

DNA polymerases of the Y family promote survival by their ability to synthesize past lesions in the DNA template. One Escherichia coli member of this family, DNA pol V (UmuC), which is primarily responsible for UV-induced and chemically induced mutagenesis, possesses a canonical beta processivity clamp-binding motif. A detailed analysis of this motif in DNA pol V (UmuC) showed that mutation of only two residues in UmuC is sufficient to result in a loss of UV-induced mutagenesis. Increased levels of wild-type beta can partially rescue this loss of mutagenesis. Alterations in this motif of UmuC also cause loss of the cold-sensitive and beta-dependent synthetic lethal phenotypes associated with increased levels of UmuD and UmuC that are thought to represent an exaggeration of a DNA damage checkpoint. By designing compensatory mutations in the cleft between domains II and III in beta, we restored UV-induced mutagenesis by a UmuC beta-binding motif variant. A recent co-crystal structure of the 'little finger' domain of E. coli pol IV (DinB) with beta suggests that, in addition to the canonical beta-binding motif, a second site of pol IV ((303)VWP(305)) interacts with beta at the outer rim of the dimer interface. Mutational analysis of the corresponding motif in UmuC showed that it is dispensable for induced mutagenesis, but that alterations in this motif result in loss of the cold-sensitive phenotype. These two beta interaction sites of UmuC affect the dual functions of UmuC differentially and indicate subtle and sophisticated polymerase management by the beta clamp.