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Site-specific mutagenesis in cells with normal DNA repair systems: transitions produced from DNA carrying a single O6-alkylguanine.

This paper describes a systematic study of transition frequencies produced in vivo when a homologous series of O6-alkylguanine residues located at a preselected position in gene G of phi X174 form I' DNA (double-stranded, circular, covalently-closed, relaxed) is transfected into spheroplasts from two strains of Escherichia coli having normal DNA repair systems. Mutant frequencies were measured as percent of total phage produced by single bursts. The results are: (A) Synthetic DNA without any alkyl group gave a transition frequency of 0.02%. (B) In E. coli AB1157, the frequencies fall into two groups depending on the alkyl group: methyl and ethyl, 8-11%; n-propyl and n-butyl approximately 0.9%. (C) The average transition frequencies were higher in AB1157 than in C600. These data demonstrate that a single O6-alkylguanine residue can produce a specific transition at significant frequencies in cells with normal repair systems and that the mutant frequency depends upon the nature of the alkyl group and the cell type.