Identification of a herpes simplex virus function that represses late gene expression from parental viral genomes.

The expression of herpes simplex virus gamma 2 (late) genes is inhibited before the onset of viral DNA replication. We report that the block in the expression of certain gamma 2 genes is relieved, at least in part, by defects in the beta ICP8 protein. We have examined the expression of the gamma 2 gene encoding glycoprotein C (gC) in cells infected with a temperature-sensitive ICP8 mutant. Under conditions in which viral DNA replication is inhibited, cells infected with the ICP8 mutant overproduce the gC family of mRNAs relative to the level observed in cells infected with a wild-type virus. The gC mRNA synthesized in cells infected with the ICP8 mutant virus is correctly initiated and spliced and is translated with the same relative efficiency as in cells infected with a replicating wild-type virus. These results suggest that ICP8 is involved in the negative regulation of gamma 2 genes expressed from parental viral genomes. The level of gC expression was greatest in cells infected with a replicating wild-type virus. These data suggest that DNA replication and genome amplification are not absolute requirements for gamma 2 gene expression but may facilitate full-level expression of these genes.