Abundant early expression of gpUL4 from a human cytomegalovirus mutant lacking a repressive upstream open reading frame

The human cytomegalovirus UL4 gene encodes a 48-kDa glycoprotein, expression of which is repressed at the translational level by a short upstream open reading frame (uORF2) within the UL4 transcript leader. Mutation of the uORF2 initiation codon in the viral genome eliminates ribosomal stalling at the uORF2 termination site, resulting in early and abundant gpUL4 protein synthesis. This mutation does not appear to affect viral replication kinetics in human fibroblasts, These results reveal that the unusual uORF2 inhibitory mechanism is a principal determinant of the abundance and timing of gpUL4 expression but is nonessential for replication in cell culture.