Expression of a uracil DNA glycosylase (UNG) inhibitor in mammalian cells: varicella-zoster virus can replicate in vitro in the absence of detectable UNG activity.

Uracil DNA glycosylase (UNG) functions as a DNA repair or proofreading enzyme. The UNG gene is present in nearly all prokaryotes and eukaryotes screened to date and is found in herpesviruses and poxviruses. Prior studies showed that viral UNG is essential for poxvirus replication. Although viral UNG is not required for herpesvirus replication, cellular UNG was thought to be essential for virus replication. To study the role of UNG in herpesvirus replication, we first showed that varicella-zoster virus (VZV) ORF59 encodes a functional UNG. We then constructed a VZV mutant with a deletion in the UNG gene and showed that the mutant was unimpaired for replication in vitro. Because cultured cells express their own endogenous UNG, we next inserted a bacteriophage UNG inhibitor UGI gene into the VZV genome. Infection of cells with VZV lacking viral UNG and expressing UGI completely abrogated detectable cellular UNG activity in vitro. Parental VZV, VZV lacking viral UNG, and VZV expressing UGI all grew to similar titers in cell culture, indicating that VZV can replicate in vitro in the absence of detectable viral or cellular UNG activity.