Role of herpes simplex virus ICP0 in the transactivation of genes introduced by infection or transfection: a reappraisal.

ICP0, a promiscuous transactivator that enhances the expression of genes introduced by infection or transfection, functions in both nucleus and cytoplasm. The nuclear functions include degradation and dispersal of ND10 bodies and suppression of silencing of viral DNA. Subsequently, ICP0 shifts to the cytoplasm. Transfection of DNA prior to infection has no effect on the localization of ICP0 in cells that are efficient expressers of transgenes (e.g., Vero and HEK293) but results in delayed cytoplasmic localization of ICP0 in cells (e.g., HEp-2 and HEL) that are poor transgene expressers. Here, we examined by real-time PCR (qPCR) the accumulation of a transgene and of viral gI mRNAs in Vero or HEp-2 cells that were transfected and then infected with wild-type or DeltaICP0 mutant viruses. The accumulation of transgene mRNA was unaffected by a DeltaICP0 mutant, gradually increased in HEp-2 cells, but increased and then decreased in Vero cells infected with wild-type virus. In both cell lines, accumulation of gI mRNA increased with time and was less affected by the transfected DNA in Vero cells than in HEp-2 cells. The relative kinetics of mRNA accumulation reflected continued synthesis and degradation of the transgene and gI mRNAs. We conclude that the role of ICP0 is to render the DNA templates introduced by transfection or infection accessible by transcriptional factors, that the two cell lines differ with respect to the transcription-ready status of entering foreign DNA in the nucleus, and that ICP0 is not per se the recruiter of transcriptional factors to the accessible DNA templates.