UNG-RPA interaction governs the choice between high-fidelity and mutagenic uracil repair

Mammalian uracil DNA glycosylase (UNG) removes uracils and initiates high-fidelity base excision repair to maintain genomic stability. During B cell development, activation-induced cytidine deaminase (AID) generates uracils in DNA, which UNG processes in an error-prone fashion to accomplish immunoglobulin (Ig) somatic hypermutation or class switch recombination (CSR). The mechanism that governs high-fidelity versus mutagenic uracil repair is not understood. The B cell tropic murine Gammaherpesvirus 68 (MHV68/MHV-4) encodes a functional homolog of UNG that can process AID-induced genomic uracils. MHV68UNG did not support hypermutation, suggesting intrinsic properties of UNG influence repair outcome. Noting the structural divergence between the UNGs, we define the RPA-interacting motif as a determinant of mutation outcome. UNG or RPA mutants unable to interact support only high-fidelity repair. In B cells, transversions at the Ig variable region are decreased, while CSR is supported. Thus UNG-RPA governs the generation of mutations and has implications for locus-specific mutagenesis in B cells and deamination-associated mutational signatures in cancer.