Effect of the R1 element on expression of the US3 and US6 immune evasion genes of human cytomegalovirus.

Abstract

Human cytomegalovirus (HCMV) has several gene products that are important for escape from immune surveillance. These viral gene products downregulate the expression of HLA molecules on the cell surface. The viral US3 and US6 gene products are expressed at immediate-early and early times after infection, respectively. There are two regulatory regions between the US3 and the US6 transcription units. The first region is an NF-kappaB responsive enhancer that promotes the immediate-early expression of the US3 gene and is designated the R2 enhancer. Upstream of the R2 enhancer is a region designated the R1 element that in transient transfection assays behaves as a silencer by repressing the effect of the enhancer on downstream gene expression (A. R. Thrower et al., J. Virol. 1996, 70, 91; Y.-J. Chan et al., J. Virol. 1996, 70, 5312). We constructed recombinant viruses with wild-type or mutated R1 elements. The expression of the US3 gene at 6 h after infection and the US6 gene at 24 h was higher when the R1 element was present. The R1 element in the context of the viral genome is not a silencer of US3 or US6 gene expression. The R1 element has multiple effects on the US3 and US6 RNAs. It enhances the level of US3 and US6 mRNA; it determines the 3'-end cleavage and polyadenylation of US6 RNA, and it stabilizes read-through viral RNAs. The potential mechanisms of R1 enhancement of US3 and US6 gene expression are discussed.