Analysis of mutations occurring during replication of a SV40 shuttle vector in mammalian cells

To analyse mutations that arise in mammalian cells we have used a SV40::plasmid shuttle vector containing a portion of the E. coli lacZ gene. We have found that following transfection into monkey Cos-7 cell mutations are not detected in the recovered plasmids at 24 h post transfection, but are found at 48 h post transfection, after the onset of DNA replication. Analysis of the mutant plasmids shows that in almost all cases the mutant phenotype is caused by a deletion or rearrangement of the lacZ gene in the shuttle vector.