Strategy for chromosomal gene targeting in RecA-deficient Escherichia coli strains.

Reengineering DNA by homologous recombination in Escherichia coli often depends on helper functions provided on a temporarily introduced replicon that is subsequently cured from the cells. The suicide vector pKSS offers a new curing strategy. pKSS specifies a variant of phenylalanyl-transfer RNA (tRNA) synthetase conferring relaxed substrate specificity towards phenylalanine analogs that results in their lethal incorporation into cellular proteins. Consequently, the presence of p-chlorophenylalanine selects for strains that have lost pKSS. This principle, in conjunction with a plasmid-borne recA gene, was exploited for targeted chromosomal mutagenesis by double homologous recombination in RecA-negative E. coli strains. Gene replacement with a kanamycin-resistance cassette was possible in a single step by plating on kanamycin and p-chlorophenylalanine agar plates and incubating at 37 degrees C. The presence of the correct chromosomal mutation and the absence of the plasmid were established by several control experiments. A simple screen confirmed the desired resistance phenotype in 44% of the initially selected clones, and 75% of these had the correct genotype.