Two processivity clamp interactions differentially alter the dual activities of UmuC

Summary 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 β 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 β can partially rescue this loss of mutagenesis. Alterations in this motif of UmuC also cause loss of the cold‐sensitive and β‐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 β, we restored UV‐induced mutagenesis by a UmuC β‐binding motif variant. A recent co‐crystal structure of the ‘little finger’ domain of E. coli pol IV (DinB) with β suggests that, in addition to the canonical β‐binding motif, a second site of pol IV ( 303 VWP 305 ) interacts with β 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 β interaction sites of UmuC affect the dual functions of UmuC differentially and indicate subtle and sophisticated polymerase management by the β clamp.