Murine cytomegalovirus open reading frame M27 plays an important role in growth and virulence in mice

Using a Tn3-based transposon mutagenesis approach, we have generated a pool of murine cytomegalovirus (MCMV) mutants. In this study, one of the mutant s, RvM27, which contained the transposon sequence at open reading frame M27 , was characterized both in tissue culture and in immunocompetent BALB/c mi ce and immunodeficient SCID mice. Our results suggest that the M27 carboxyl -terminal sequence is dispensable for viral replication in vitro. Compared to the wild-type strain and a rescued virus that restored the M27 region, R vM27 was attenuated in growth in both BALB/c and SCID mice that were intrap eritoneally infected with the viruses. Specifically, the titers of RvM27 in the salivary glands, lungs, spleens, livers, and kidneys of the infected S CID mice at 21 days postinfection were 50- to 500-fold lower than those of the wild-type virus and the rescued virus. Moreover, the virulence of the m utant virus appeared to be attenuated, because no deaths occurred among SCI D mice infected with RvM27 for up to 37 days postinfection, while all the a nimals infected with the wild-type and rescued viruses died within 27 days postinfection. Our observations provide the first direct evidence to sugges t that a disruption of M27 expression results in reduced viral growth and a ttenuated viral virulence in vivo in infected animals. Moreover, these resu lts suggest that M27 is a viral determinant required for optimal MCMV growt h and virulence in vivo and provide insight into the functions of the M27 h omologues found in other animal and human CMVs as well as in other betaherp esviruses.