Regulation of cytomegalovirus late gene expression: gamma genes are controlled by posttranscriptional events.

We investigated the control of human cytomegalovirus (CMV) late (gamma)-gene expression in human fibroblast cells. Transcriptional activity of two gamma genes, encoding ICP27, a structural component (matrix or tegument) of virions, and ICP36, a major DNA-binding protein family, was followed by analysis of steady-state RNA levels during viral infection. Synthesis of the protein products of these genes was analyzed with specific monoclonal antibodies in conjunction with sensitive immunoblot or immunoprecipitation analysis. Although accumulation of ICP27 and ICP36 was not abundant until late times, both late genes were as transcriptionally active at early times (4 h postinfection) as at late times (48 h postinfection). Reduced amounts (less than 5% of late levels) of the protein products were detected at early times, demonstrating that a small proportion of the ICP27 and ICP36 RNA made at this time was translated. These observations establish that expression of at least two CMV gamma genes is regulated through posttranscriptional events. The very early transcriptional activation of late genes and the relative importance of posttranscriptional regulation to late-gene expression distinguishes CMV from other well-studied herpesviruses and does not appear analogous to late-gene regulation in any other DNA animal virus.