High mutation frequency in DNA transfected into mammalian cells.

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Abstract
The lacI gene of Escherichia coli was used to score mutation in mammalian cells of SV40-based recombinant DNA vectors that provide for replication and selection in both bacterial and mammalian cells. Plasmid DNA was introduced into African green monkey kidney COS7 cells by DEAE-dextran transfection, allowed to replicate in the mammalian cells, and then returned to E. coli for analysis. Mutants in lacI were observed at frequencies of one to several percent, compared with a spontaneous mutation rate in E. coli of < 10-5. The lesions include a large number of base substitutions, in addition to deletions, duplications and more complex rearrangements, including insertion into the plasmid of sequences originating in the host genome. Possible sources of the high mutation frequency and its implications for experiments involving DNA transfer are discussed.