The varicella-zoster virus (VZV) open reading frame 54 (ORF54) gene encodes an 87-kDa monomer that oligomerizes to form the VZV portal protein, pORF54. pORF54 was hypothesized to perform a function similar to that of a previously described herpes simplex virus 1 (HSV-1) homolog, pUL6. pUL6 and the associated viral terminase are required for processing of concatemeric viral DNA and packaging of individual viral genomes into preformed capsids. In this report, we describe two VZV bacterial artificial chromosome (BAC) constructs with ORF54 gene deletions, ⌬54L (full ORF deletion) and ⌬54S (partial internal deletion). The full deletion of ORF54 likely disrupted essential adjacent genes (ORF53 and ORF55) and therefore could not be complemented on an ORF54-expressing cell line (ARPE54). In contrast, ⌬54S was successfully propagated in ARPE54 cells but failed to replicate in parental, noncomplementing ARPE19 cells. Transmission electron microscopy confirmed the presence of only empty VZV capsids in ⌬54S-infected ARPE19 cell nuclei. Similar to the HSV-1 genome, the VZV genome is composed of a unique long region (U L ) and a unique short region (U S ) flanked by inverted repeats. DNA from cells infected with parental VZV (VZV LUC strain) contained the predicted U L and U S termini, whereas cells infected with ⌬54S contained neither. This result demonstrates that ⌬54S is not able to process and package viral DNA, thus making pORF54 an excellent chemotherapeutic target. In addition, the utility of BAC constructs ⌬54L and ⌬54S as tools for the isolation of site-directed ORF54 mutants was demonstrated by recombineering single-nucleotide changes within ORF54 that conferred resistance to VZV-specific portal protein inhibitors.
IMPORTANCE
Antivirals with novel mechanisms of action would provide additional therapeutic options to treat human herpesvirus infections.Proteins involved in the herpesviral DNA encapsidation process have become promising antiviral targets. Previously, we described a series of N-␣-methylbenzyl-N=-aryl thiourea analogs that target the VZV portal protein (pORF54) and prevent viral replication in vitro. To better understand the mechanism of action of these compounds, it is important to define the structural and functional characteristics of the VZV portal protein. In contrast to HSV, no VZV mutants have been described for any of the seven essential DNA encapsidation genes. The VZV ORF54 deletion mutant described in this study represents the first VZV encapsidation mutant reported to date. We demonstrate that the deletion mutant can serve as a platform for the isolation of portal mutants via recombineering and provide a strategy for more in-depth studies of VZV portal structure and function.H erpesvirus DNA encapsidation is understood only in general terms. The mechanistic events and the function(s) of the proteins involved are still under investigation. During viral replication, progeny DNA genomes are synthesized in the nucleus as long, branched head-to-tail concatemers. The capsid proteins are synt...