A Positive/Negative Selection Cassette for Red Recombination of BAC Clones

Conventional gene cloning methods always require tedious processes confined by PCR conditions and restriction sites, which might result in a large number of redundant fragments in the constructs. These shortages can be overcome by using DNA recombination methods. Homologous recombination in Escherichia coli cells was used to develop positive/negative cassette for phage λ red operon recombination. In the bi-functional cassette, the Ala294Gly α-subunit mutant of phenylalanyl-tRNA synthetase (mPheS) was fused with the kanamycin-resistant (KanR) cassette under the control of the promoter of the E. coli gene NEOKAN as a single open reading frame (ORF) by PCR. KanR was utilized as a positive selection marker, while the mutant gene mPheS served as a negative selection marker. Two-step red-mediated recombination was used to replace the third exon of TMEM18 in the BAC clone RP23-25C13 with eGFP. Initially, the third TMEM18 exon in the BAC was replaced with the NEOKAN-mPheS-KanR cassette, and 40% of the clones on the positive selection plate (LB plate with kanamycin) were positive. The second step involved replacing the NEOKAN-mPheS-KanR cassette with eGFP in the modified BAC, and 60% of the clones on the negative plate (p-Cl-phe plate) were positive. Following the two steps, eGFP was used to replace the third exon of TMEM18 in the BAC. The NEOKAN-mPheS-KanR could be a promising choice as a positive/negative selection cassette for red recombination to establish constructs and make gene manipulation on BAC more applicable and effortless.