The herpes simplex virus host shutoff RNase degrades cellular and viral mRNAs made before infection but not viral mRNA made after infection.

A herpes simplex virus tegument protein brought into the cell during infection and designated the virion host shutoff protein (VHS) is an endoribonuclease that degrades mRNA. The prevailing view for many years has been that the VHS-RNase does not discriminate between cellular and viral RNAs and that the viruses prevail because the accumulation of viral transcripts outpaces their degradation. Here we report the following. (i) The degradation of viral mRNA made during infection of Vero or HEp-2 cells proceeds at a much-reduced rate compared to that of cellular mRNA. In effect, viral mRNAs are largely stable, whereas cellular mRNAs are rapidly degraded or, in the case of AU-rich mRNA, cleaved and rendered dysfunctional. (ii) In contrast to viral mRNAs made after infection, viral mRNAs expressed by plasmids transfected into cells prior to infection are degraded after infection at a rate comparable to that of cellular mRNAs. Moreover, the mRNA encoded by the transfected plasmid is hyperadenylated in the infected cell. Hyperadenylation but not degradation of mRNAs is blocked by actinomycin D. The results indicate that VHS-mRNA discriminates between viral and cellular mRNA but only in the context of infection and that discrimination is not based on the sequence of the mRNA but most likely on one or more viral factors expressed in the infected cell.