Abstract
A remarkably high percentage of proto-oncogene, growth factor, cellular receptor, and viral transcript leaders contain short upstream open reading frames (uORFs), yet the significance and regulatory effects of these uORFs have not been well characterized. In the case of the human cytomegalovirus gpUL4 (gp48) transcript, the second of three uORFs (uORF2) inhibits translation of the downstream cistron by a process that depends on the uORF2 amino acid coding information. To investigate the mechanism underlying this unusual regulatory element, we adapted the toeprinting (or reverse transcriptase extension inhibition) assay for use in detecting positions of ribosomal stalling on gp48 transcripts. Using a cell-free translation system, we demonstrate that ribosomes arrest at the termination codon of uORF2 by a uORF2 coding sequence-dependent mechanism. Further, the sequence requirements for ribosomal stalling are the same as for inhibition of downstream translation. We also provide evidence for ribosomal stalling in vivo, on the natural viral mRNA. These data support the hypothesis that the inhibition of downstream translation results from uORF2 peptide-dependent ribosomal arrest at termination and suggest that translation termination may be a regulatory step in expression of some eukaryotic genes.
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