Rapid and efficient site-specific mutagenesis without phenotypic selection.

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Abstract
Several single-base substitution mutations have been introduced into the lacZ alpha gene in cloning vector M13mp2, at 40-60% efficiency, in a rapid procedure requiring only transfection of the unfractionated products of standard in vitro mutagenesis reactions. Two simple additional treatments of the DNA, before transfection, produce a site-specific mutation frequency approaching 100%. The approach is applicable to phenotypically silent mutations in addition to those that can be selected. The high efficiency, approximately equal to 10-fold greater than that observed using current methods without enrichment procedures, is obtained by using a DNA template containing several uracil residues in place of thymine. This template has normal coding potential for the in vitro reactions typical of site-directed mutagenesis protocols but is not biologically active upon transfection into a wild-type (i.e., ung+) Escherichia coli host cell. Expression of the desired change, present in the newly synthesized non-uracil-containing covalently closed circular complementary strand, is thus strongly favored. The procedure has been applied to mutations introduced via both oligonucleotides and error-prone polymerization. In addition to its utility in changing DNA sequences, this approach can potentially be used to examine the biological consequences of specific lesions placed at defined positions within a gene.