Characterization of a potent varicella-zoster virus-encoded trans-repressor

Using a transient expression assay in Vero cells, we have shown that the protein product from gene 61 of varicella-zoster virus (VZV) can repress the function of the VZV encoded trans-activators on putative viral immediate-early, early, and late gene promoters. The repression is exerted at the transcriptional level and requires functional gene 61 protein. This trans-repressor is the herpes simplex type 1 ICP0 (a trans-activator) homolog, as defined by gene location, the sharing of a cysteine-rich putative zinc-binding finger in the amino-terminal region, and limited amino acid homology. Open reading frame 61 (ORF61)-mediated trans-repression appears to be specific for VZV-encoded trans-activators in that it has no effect on simian virus 40 and Rous sarcoma virus promoters. Moreover, it does not inhibit trans-activation of the human T-lymphotropic virus type I and human immunodeficiency virus long terminal repeats by tax and tat genes, respectively. We constructed plasmids with mutations in ORF61 and tested them for their ability to inhibit trans-activator (VZV genes 4 and 62)-mediated activation of the viral thymidine kinase promoter-chloramphenicol acetyltransferase construct. Mutants containing interruptions in ORF61 lost their trans-repressing ability, as demonstrated at both the protein and steady-state RNA levels. These results suggest that the ORF61 protein product can mediate down-regulation of VZV gene expression.