Control of mRNA stability by the virion host shutoff function of herpes simplex virus

vhs1 is a mutant of herpes simplex virus type 1 that is defective in the virion host shutoff function responsible for the degradation of cellular mRNAs and the concomitant shutoff of host protein synthesis. In this study, the effect of the vhs1 mutation on the metabolism of viral mRNAs was examined by measuring the half-lives and patterns of accumulation of 10 different viral mRNAs representing all kinetic classes. The vhs1 mutation had the effect of dramatically lengthening the cytoplasmic half-lives of all 10 mRNAs. In wild-type virus infections, the 10 mRNAs had similar half-lives, suggesting that little, if any, target mRNA selectivity was exhibited by the vhs function. The vhs1 mutation caused overaccumulation of a number of mRNAs. The effect was most dramatic for the alpha (immediate-early) mRNA for ICP27 and the beta (early) mRNAs encoding thymidine kinase, ICP8, and DNA polymerase. Whereas in wild-type infections these mRNAs increased to peak levels and subsequently declined in abundance, in vhs1 infections they continued to accumulate until late times. A significant but less dramatic overaccumulation was observed for several beta-gamma (delayed-early) and gamma (late) mRNAs. The results suggest that the vhs protein plays an important role in determining the half-lives of viral mRNAs belonging to all kinetic classes and in so doing is important in the normal downregulation at late times of alpha and beta gene expression.