Herpes simplex virus type 1 ICP0 regulates expression of immediate-early, early, and late genes in productively infected cells

Varicella-zoster virus open reading frame 61 protein is functionally homologous to herpes simplex virus type 1 ICP0

The varicella-zoster virus (VZV) open reading frame 61 (ORF61) protein is thought to be the homolog of herpes simplex virus type 1 (HSV-1) ICP0, based on gene location and limited amino acid homology. However, HSV-1 ICP0 trans activates HSV-1 genes, while VZV ORF61 protein trans represses the function of VZV trans activators on VZV promoters in transient expression assays. To investigate the functional relatedness of HSV-1 ICP0 and VZV ORF61 protein, we established Vero and MeWo cell lines which stably express VZV ORF61 under the control of a metallothionein promoter and performed complementation studies with an HSV-1 ICP0 deletion mutant (7134). Mutant 7134 is impaired for plaque formation and replication at a low multiplicity of infection in cell culture, but these defects were complemented by up to 200-fold in Vero cell lines expressing VZV ORF61. Likewise, the efficiency of plaque formation was improved by up to 100-fold in MeWo cell lines expressing VZV ORF61. A cell line expressing another VZV immediate-early gene product (ORF62) was unable to complement mutant 7134. HSV-1 mutants which are deleted for other HSV-1 immediate-early gene products (ICP4, ICP27) were unable to grow in VZV ORF61-expressing cell lines. These results indicate that, despite marked differences in their sequences and in effects on their cognate promoters in transient expression assays, VZV ORF61 protein is the functional homolog of HSV-1 ICP0.

Multimerization of ICP0, a herpes simplex virus immediate-early protein

ICP0, a herpes simplex virus immediate-early gene product, is a highly phosphorylated nuclear protein that is a potent activator of virus and host genes. Using biochemical and genetic assays employing plasmids encoding mutant forms of ICP0 and a recombinant adenovirus that expresses ICP0, we mutant forms of ICP0 and a recombinant adenovirus that expresses ICP0, we provide evidence that the protein multimerizes. Some mutant forms of ICP0 were transdominant and interfered with activation of a target reporter gene or with complementation of an ICP0-minus virus.

Herpes simplex viruses with mutations in the gene encoding ICP0 are defective in gene expression

Herpes simplex virus type 1 (HSV-1) mutants with codon insertions and deletions in IE-0, the gene encoding ICP0, were constructed. The HSV-1 deletion mutant dl1403 (N. D. Stow and E. C. Stow, J. Gen. Virol. 67:2571-2585, 1986) and an IE-0:lacZ transplacement vector isolated in this study were used to facilitate the construction of mutant viruses. Mutant viruses, all of which produced stable ICP0, were examined for their ability to plaque and grow on both Vero and HeLa cells because previous results showed that HSV-1 immediate-early (IE) gene promoters and their products are differentially expressed in these cells (J. Chen, X. Zhu, and S. Silverstein, Virology 180:207-220, 1991; I. H. Gelman and S. Silverstein, J. Virol. 61:2286-2296, 1987). Viruses with IE-0 genes that only poorly activated reporter genes in transient expression assays plaqued less efficiently on Vero cells and consistently accumulated decreased levels of late proteins. These mutants were also examined in single-step growth curve experiments and for the dependence of virus yield on multiplicity of infection (MOI). At low MOIs, their yields were less in Vero cells than in HeLa cells; by contrast, at high MOIs, there was no apparent difference in yield in either cell type, although each virus produced considerably fewer progeny than wild-type virus. Analysis of steady-state levels of RNA from genes representing each of the three major kinetic classes demonstrated that lower levels of RNAs accumulate in these mutants. We conclude from these studies that while ICP0 is not essential for virus growth in tissue culture, defects in this gene result in impairment of virus replication and delay the expression of early and late gene transcripts.