Human cytomegalovirus uracil DNA glycosylase is required for the normal temporal regulation of both DNA synthesis and viral replication.

Human cytomegalovirus (CMV) encodes a gene, UL114, whose product is homologous to the uracil DNA glycosylase and is highly conserved in all herpesviruses. This DNA repair enzyme excises uracil residues in DNA that result from the misincorporation of dUTP or spontaneous deamination of cytosine. We constructed a recombinant virus, RC2620, that contains a large deletion in the UL114 open reading frame and carries a 1.2-kb insert containing the Escherichia coli gpt gene. RC2620 retains the capacity to replicate in primary human fibroblasts and reaches titers that are similar to those produced by the parent virus but exhibits a significantly longer replication cycle. Although the rate of expression of alpha and beta gene products appears to be unaffected by the mutation, DNA synthesis fails to proceed normally. Once initiated, DNA synthesis in mutant virus-infected cells proceeds at the same rate as with wild-type virus, but initiation is delayed by 48 h. The mutant virus also exhibits two predicted phenotypes: (i) hypersensitivity to the nucleoside analog 5-bromodeoxyuridine and (ii) retention of more uracil residues in genomic DNA than the parental virus. Together, these data suggest UL114 is required for the proper excision of uracil residues from viral DNA but in addition plays some role in establishing the correct temporal progression of DNA synthesis and viral replication. Although such involvement has not been previously observed in herpesviruses, a requirement for uracil DNA glycosylase in DNA replication has been observed in poxviruses.