Deletion of the Herpes Simplex Virus 1 U L 49 Gene Results in mRNA and Protein Translation Defects That Are Complemented by Secondary Mutations in U L 41

Herpes simplex virus 1 (HSV-1) virions, like those of all herpesviruses, contain a protein layer termed the tegument localized between the capsid and the envelope. VP22, encoded by the U L 49 gene, is one of the most abundant tegument proteins in HSV-1 virions. Studies with a U L 49-null mutant showed that the absence of VP22 resulted in decreased protein synthesis at late times in infection. VP22 is known to form a tripartite complex with VP16 and vhs through direct interactions with VP16. Given that U L 49-null mutants have been shown to acquire spontaneous secondary mutations in the U L 41 gene, which encodes vhs, we hypothesized that VP22 and vhs may play antagonistic roles during HSV-1 infections. In the present study, we show that the protein synthesis defect observed in U L 49-null virus infections was rescued by a secondary, compensatory frameshift mutation in U L 41. A double mutant bearing a deletion of U L 49 and a point mutation in vhs previously shown to specifically abrogate vhs's RNase activity also resulted in a rescue of protein synthesis. To determine whether the U L 49 − protein synthesis defect, and the rescue by secondary mutations in vhs, occurred at the mRNA and/or translational levels, quantitative reverse transcriptase PCR (qRT-PCR) and polysome analyses were performed. We found that the absence of VP22 caused a small decrease in mRNA levels as well as a defect in polysome assembly that was independent of mRNA abundance. Both defects were complemented by the secondary mutations in vhs, indicating functional interplay between VP22 and vhs in both accumulation and translation of viral mRNAs.