Enzymatic techniques for the isolation of random single-base substitutions in vitro at high frequency.

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Abstract
A general and efficient method was developed to generate large numbers of single-base substitution mutations simply and rapidly. A unique f1 phage recombinant DNA clong vector is described, which contains the .vphi.X174 origin of viral strand DNA synthesis and allows one to direct mutagenesis to any specific segment of DNA. Gapped circular DNA is constructed by annealing viral single-stranded circular DNA [ss(c) DNA] with a mixture of linear duplex DNAs that have had their 3''-OH termini processively digested with Escherichia coli exonuclease III under conditions in which the resulting, newly generated 3''-OH termini present in the various hybrid molecules span the region of interest. Base changes are induced by misincorporation of an .alpha.-thiodeoxynucleoside triphosphate analog onto this primer-temperate, followed by DNA repair synthesis. The asymmetric segregation of mutants from wild-type sequences is accomplished by double-stranded replicative form DNA .fwdarw. ss(c) DNA synthesis in vitro, initiated from the .vphi.X174 viral strand origin sequence present on the vector DNA. Mutated ss(c) DNA is screened by the dideoxy chain termination method. In 1 mutagenesis experiment, 21 independent single-base substitutions were isolated in a 72-nucleotide-long target region. DNA sequence analysis showed that all possible base transversions and transitions were represented.