Uracil-DNA Glycosylase (UNG)-Deficient Mice Reveal a Primary Role of the Enzyme during DNA Replication

Gene-targeted knockout mice have been generated lacking the major uracil-DNA glycosylase, UNG. In contrast to ung − mutants of bacteria and yeast, such mice do not exhibit a greatly increased spontaneous mutation frequency. However, there is only slow removal of uracil from misincorporated dUMP in isolated ung −/− nuclei and an elevated steady-state level of uracil in DNA in dividing ung −/− cells. A backup uracil-excising activity in tissue extracts from ung null mice, with properties indistinguishable from the mammalian SMUG1 DNA glycosylase, may account for the repair of premutagenic U:G mispairs resulting from cytosine deamination in vivo. The nuclear UNG protein has apparently evolved a specialized role in mammalian cells counteracting U:A base pairs formed by use of dUTP during DNA synthesis.