Mutations affecting the cytoplasmic domain of the Moloney murine leukemia virus envelope protein: rapid reversion during replication.

Five premature termination mutations and five missense mutations were introduced into the portion of cloned Moloney murine leukemia virus (M-MuLV) DNA encoding the Env cytoplasmic domain. All of the mutant DNAs gave rise to replication-competent virus after transfection of NIH/3T3 cells, but several of the mutant DNAs scored as replication-defective when introduced into Rat2 cells. Cell lines stably expressing the mutant DNAs all released virion particles, and in all but one case infectious virus were generated. These viable mutants were all found to have reverted to the wild-type sequence. To generate fully mutant virus stocks, the mutant DNAs were introduced transiently into COS cells, which are resistant to infection with MuLV, thus prohibiting reversion by error-prone mechanisms involving reverse transcription. Virions harvested from the COS cells were confirmed as mutant by analyzing both virion proteins and the viral DNA they generated, and were then tested for infectivity in NIH/3T3 cells. The mutant viruses were infectious, but still rapidly gave rise to revertants. We conclude that the mutations within the cytoplasmic domain do not provide an absolute block to virus replication, but that the mutants replicate more slowly than the wild-type and quickly give rise to revertants with selective advantage for replication.