A simple vector modification to facilitate oligonucleotide-directed mutagenesis.

We describe a simple modification of commonly used single-stranded cloning vectors that permits the efficient recovery of mutant DNA molecules in oligonucleotide-directed mutagenesis experiments, even when the absolute efficiency of mutagenesis is very low. The modification consists of the insertion of a short synthetic DNA fragment into the vector's polylinker and permits the identification of mutant clones based on a standard chromogenic plate assay for bacterial colonies or phage plaques producing functional beta-galactosidase. Other useful properties of the original vector are retained in the modified version. In vitro mutagenesis reactions are carried out with two oligonucleotides, one to introduce the mutation of interest, and the second to correct a frameshift mutation introduced into the beta-galactosidase gene of the modified vector. We have found that these two sequence changes are closely linked following transformation of an appropriate E. coli strain with the products of the in vitro mutagenesis reaction, and have thereby recovered desired mutations at a frequency of about 50% even when the overall mutagenesis efficiency is less than 1%. By alternately correcting and re-introducing the beta-galactosidase frameshift mutation, we have shown that multiple rounds of mutagenesis can be carried out on the same template with a high efficiency of mutant recovery in each step. Modifications similar or identical to those we describe here should be feasible for most commonly used single-stranded cloning vectors and should increase the usefulness of these vectors by providing an additional option for oligonucleotide-directed mutagenesis to be used in conjunction with or in lieu of other commonly used approaches.