In vitro models of mutagenesis.

The bypass of lesions in DNA with insertion of nucleotides opposite damaged bases has been studied as a model for mutagenesis in an in vitro system. Lesions introduced by dimethyl sulfate at adenines and by ultraviolet light at pyrimidine dimers act as termination sites on both double- and single-stranded DNA templates. Base selection opposite noninformational lesions is, in part, a property of the polymerases: different polymerases have different selectivities although all polymerases tested seem to prefer purines. The ability to insert "incorrect" bases is determined in part by the sequence 5' to the lesion on the template strand. The hypothesis that damaged purines tend to result in transversions can be applied to published data on activation of the c-ras oncogene.