Selective Uptake of Small RNA Molecules in the Virion of Murine Gammaherpesvirus 68

Noncoding RNAs are a feature of many herpesvirus genomes. They include microRNAs, whose function is the subject of intense investigation, in addition to longer RNA molecules such as the Epstein-Barr virus-encoded RNAs and herpesvirus saimiri U RNAs, which have been known for some time but whose function is still not well defined. Murine gammaherpesvirus 68 (MHV-68) encodes eight viral tRNA-like molecules (vtRNAs) of unknown function. Investigating the kinetics of expression of the vtRNAs, we observed that they were present directly after infection with the virus. This strongly suggested that vtRNAs were part of the virion structure, which was confirmed by their detection within various purified, RNase-treated preparations. Although both viral and cellular mRNAs were also detected within the MHV-68 virion, the major RNA species present were small RNAs of around 70 nucleotides in length. Interestingly, incorporation of viral mRNA was not related to the relative abundance in infected cells, as M11 mRNA, which is present at low abundance, was found in virions. MHV-76, which lacks the genes encoding the vtRNAs, also incorporated small RNA molecules within the virion, suggesting a requirement for these molecules for virion maturation. In productively infected cells the vtRNAs localized predominantly within the cytoplasm, although they also exhibited a globular pattern of nuclear staining. Their presence in the cytoplasm is consistent with interaction with virion components prior to maturation of virus particles. The significance of these findings for virion architecture and function is discussed.