Varicella-zoster virus gene 63: transcript mapping and regulatory activity.

The varicella-zoster virus (VZV) putative immediate-early (IE) protein encoded by ORF63 is the homolog of HSV-1 ICP-22. To further characterize ORF63 and its function, Northern analysis, primer extension, and S1 nuclease assays were used to map its transcripts, and transient transfection assays were performed with constructs containing ORF63 or its promoter region. Two transcripts were identified: a 0.9-kb transcript spans ORF63 alone, and a 1.8-kb transcript reads through ORF64. Two prominent transcription start sites were identified at -88 and -157 relative to the ORF63 ATG, and two potential TATA elements were identified. In transient transfection assays, the 63 promoter was weakly activated by VZV ORF4 and ORF62 under their homologous promoters, was more strongly activated by ORF62 under the control of a constitutive CMV promoter, and was synergistically activated by ORFs 4 and 62 together. ORF63, driven by its own or by a heterologous SV40 promoter, exerted minimal effects on diverse VZV putative IE and early promoters, showed no clear evidence of autoregulation, and did not directly inhibit the ORF62 promoter as had been reported previously. ORF63's behavior in transient assays suggests that it plays only a limited regulatory role in modulating VZV gene expression.