Abstract: A PCR fragment including 30nt of flanking DNA homologous to the chromosome in each end and a antibiotic－resistant gene in the middle is introduced into the host cell of BW25113／ pIJ790. The three proteins of λ phage－－－Exo、Bet and Gam, expressed by pIJ790, composing the Red recombination system has the advantage of high one－step recombination rate of linear fragment. The antibiotic－resistant gene replaces the target gene as a result. This research obtaines a derivative of E. coli whose efflux pump gene acrAB is deleted. The mutant strain will help to research of mechanism of antimicrobial materials.

Key words: Red recombination, PCR, gene knock－out, efflux pump gene