Translational control of cytomegalovirus gene expression is mediated by upstream AUG codons.

Human cytomegalovirus regulates gene expression at both the transcriptional and posttranscriptional levels. We have characterized posttranscriptional signals carried on the RNA of one beta (or delayed-early) gene, signals that we show delay the translation of an actively transcribed gene. The cis-acting components of this signal map to two upstream AUG codons in the 5' leader of the beta gene transcript that act to delay the kinetics and reduce the levels of gene expression. Both upstream AUG codons are part of short open reading frames, and the presence of either is sufficient to suppress expression of a downstream open reading frame. These upstream natural AUG codons appear to act in cis at the level of translation, consistent with the scanning hypothesis of eucaryotic translation. The occurrence of natural upstream AUG codons in other cytomegalovirus transcripts suggests their physiological significance in influencing the kinetics and steady-state levels of gene expression during viral growth.