Site-specific mutagenesis using synthetic oligodeoxyribonucleotide primers: II. In vitro selection of mutant DNA.

A method for the in vitro selection of mutant DNA has been devised as an adjunct to the recently developed method for the use of short enzymatically-synthesized oligodeoxyribonucleotides of defined sequence as site-specific mutagens for circular DNA. The selection method uses the mutating oligodeoxyribonucleotide as a primer for Escherichia coli DNA polymerase I (large fragment) under conditions where there is preferential interaction with mutant DNA template. After ligation using T4 DNA ligase, endonuclease S1 is used to degrade single-stranded non-mutant DNA leaving the desired mutant as closed circular duplex DNA. This paper describes the development of the method using mutants in phi X174 DNA as the model system. Studiies on the changes A leads to G and G leads to A at position 587 of phi X174 viral DNA (am3 to wild-type and its reversal) show that one or two cycles of selection can lead to a population of phage consisting of close to 100% mutants.