In vitro and in vivo characterization of a murine cytomegalovirus with a transposon insertional mutation at open reading frame M43

We have recently generated a pool of murine cytomegalovirus (MCMV) mutants by using a Tn3-based transposon mutagenesis approach. In this study, one of the MCMV mutants, RvM43, which contained the transposon inserted in open r eading frame M43, was characterized. Our results provide the first direct e vidence to suggest that M43 is not essential for viral replication in vitro in NIH 3T3 cells, Moreover, RvM43 exhibited a titer similar to that of the wild-type virus in the Lungs, livers, spleens, and kidneys of both BALB/c and SCID mice and was as virulent as the wild-type virus in killing SCID mi ce that had been intraperitoneally infected with the viruses. In contrast, titers of the mutant virus in the salivary glands of the infected animals a t 21 days postinfection were significantly (100 to 1,000-fold) lower than t hose of the wild-type virus and a rescued virus that restored the M43 regio n and its expression. Thus, M43 appears to be not essential for viral growt h in vivo in the lungs, livers, spleens, and kidneys of infected animals an d is also dispensable for virulence in killing SCID mice. Moreover, our res ults suggest that M43 is an MCMV determinant for growth in the salivary gla nds. Studies of viral genes required for replication in the salivary glands are important in understanding the mechanism of viral tropism for the sali vary glands and shedding in saliva, which is believed to be one of the majo r routes of CMV transmission among healthy human populations.