Translational regulation of the human cytomegalovirus pp28 (UL99) late gene

The pp28 (UL99) gene of human cytomegalovirus is expressed as a true late gene, in that DNA synthesis is absolutely required for mRNA expression. Our previous studies demonstrated that pp28 promoter sequences from position -40 to +106 are sufficient for late gene expression in the context of the viral genome (C. P. Kohler, J. A. Kerry, M. Carter, V. P. Muzithras, T. R. Jones, and R. M. Stenberg, J. Virol. 68:6589-6597, 1994). To extend these studies, we have examined the sequences in the downstream leader region of the pp28 gene for their role in late gene expression. Deletion of sequences from position -6 to +46 (deltaSS) results in a threefold increase in gene expression in transient assays. In contrast, deletion of sequences from position +46 to +88 (deltaA) has little effect on gene expression. These results indicate that the sequences from position -6 to +46 may repress gene expression. To further analyze this region, site-directed mutagenesis was performed. Mutation of residues from either position +1 to +6 (SS1) or position +12 to +17 (SS2) duplicated the effect of the deltaSS deletion mutant, indicating that sequences from position +1 to +17 were important for the inhibitory effect. To assess the biological significance of these events, a recombinant virus construct containing the deltaSS mutant promoter regulating expression of the chloramphenicol acetyltransferase (CAT) reporter gene was generated. Analysis of this virus (RV delta SSCAT) revealed that deletion of sequences from position -6 to +46 does not alter the kinetic class of this promoter. However, the ratio of CAT protein to CAT mRNA levels in RV delta SSCAT-infected cells was 8- to 12-fold higher than that observed in the parental RV24/26CAT-infected cells. These results imply that the leader sequences within the pp28 gene can regulate the translation of this late gene.