Human cytomegalovirus UL36-38 and US3 immediate-early genes: temporally regulated expression of nuclear, cytoplasmic, and polysome-associated transcripts during infection

Abstract

During permissive in vitro infection, the human cytomegalovirus (HCMV) UL36-38 and US3 immediate-early (IE) regions give rise to multiple distinct species of RNA in a temporally regulated manner. We have compared the temporally regulated expression of the UL36-38 and US3 regions with that of the well-characterized major IE (MIE) region. Northern (RNA) blot hybridizations with antisense RNA probes were used to examine RNA isolated from infected cells at IE, early, and late times after infection and from cells infected in the presence of anisomycin (used to block de novo viral protein synthesis) or in the presence of phosphonoformate (used to block HCMV DNA synthesis). Different US3 region transcripts were expressed in the cytoplasm during the IE and late phases of infection, with kinetics similar to those of the MIE region. In contrast, various cytoplasmic transcripts from the UL36-38 region were expressed during each of the IE, early, and late phases of infection, including some expressed from IE through late times. The levels of steady-state RNA from the US3 and MIE regions were increased dramatically by infection in the presence of anisomycin, predominantly because of an increase in multiply spliced transcripts. Two of the three UL36-38 IE transcripts were largely unaffected by anisomycin and were expressed abundantly throughout infection, but a third, multiply spliced UL36-38 IE transcript was abundant only during infection in the presence of anisomycin. Nuclear, cytoplasmic, and polysome-associated transcripts from the three IE regions were not significantly different qualitatively or quantitatively. These results suggest that posttranscriptional controls at the levels of nuclear retention or polysome exclusion of transcripts are not operative for the IE region genes. Overall, these results indicate common features of expression of US3, MIE, and UL36-38, in addition to distinctive expression of the UL36-38 region during all temporal phases of expression.