Herpes simplex virus 1 ICP27 is required for transcription of two viral late (gamma 2) genes in infected cells.

The herpes simplex virus infected cell protein 27 (ICP27) is required for the expression of certain early viral proteins and for many late proteins during productive infection. Expression of at least one late (gamma 2) gene, that encoding glycoprotein C, is severely restricted in the absence of functional ICP27. The exact mode of action by which ICP27 induces late gene expression is not known, but the effect is apparent at the mRNA level as demonstrated by Northern blot analysis. To determine whether ICP27 activates late genes via transcriptional or posttranscriptional mechanisms, we initially used nuclear run-on assays to measure transcription of viral genes in Vero cells infected with wild-type (WT) virus or an ICP27 nonsense mutant virus, n504. We observed a 4-fold reduction in the nuclear run-on signal from the coding strand of the gC gene for n504-infected cells compared to that of WT-infected cells. However, interpretation of the results was complicated by the observation of a significant signal from the noncoding strand in these experiments. To obviate the problem of symmetrical transcription, we utilized in vivo RNA pulse-labeling to measure the amount of transcription of viral genes in cells infected with either WT virus or n504 virus. We found a 5- to 10-fold reduction in the transcription of the gC and U(L)47 genes, two late genes, in cells infected with n504 compared to that in cells infected with WT virus. In contrast, transcription of the ICP8 gene, an early gene, was similar in WT and n504 virus-infected cells. We also examined the stability of the gC and U(L)47 gene transcripts in n504-infected cells, and we found it to be comparable to that in WT virus-infected cells, further supporting an effect on transcription. Transcription of the gC and U(L)47 genes by n504 was normal in a cell line that expresses WT ICP27. From these results we conclude that ICP27 is required for transcription of the late gC and U(L)47 genes during productive infection.