Translational regulation of herpes simplex virus DNA polymerase

Using as antigens fusion proteins expressed in bacteria, we have generated polyclonal antisera specific for the herpes simplex virus (HSV) type 1 DNA polymerase. A variety of immunologic, genetic, and biochemical assays were used to characterize these antisera and demonstrate their specificity for the HSV DNA polymerase. Using these antisera, measurements of the synthesis and accumulation of HSV DNA polymerase in infected Vero cells were made. Peak rates of polymerase synthesis were observed at 4 h postinfection, as much as 2 h before peak levels of polymerase mRNA accumulation. At all times examined, the HSV DNA polymerase polypeptide was found to be synthesized at a lower rate per mRNA than the viral thymidine kinase, with this difference being especially dramatic at later times. Infected-cell RNA isolated at 2 and 6 h postinfection directed the synthesis of similar amounts of polymerase polypeptide per polymerase transcript in rabbit reticulocyte lysates, indicating that polymerase transcripts are inherently as translatable at both times. An HSV mutant in which sequences including a short upstream open reading frame in the HSV DNA polymerase transcript were deleted specified polymerase mRNA whose translational efficiency was no more than marginally greater than that of the wild-type virus. These results demonstrate that polymerase expression is regulated by inefficient translation mediated by sequences other than the short upstream open reading frame and that this leads to an early shutoff of polymerase synthesis during HSV infection.