Single Step (KRX) Competent Cells Technical Bulletin

Literature # TB352

Single Step (KRX) Competent Cells are prepared from the KRX strain, an E. coli K12 derivative with salient features associated with cloning and screening strains plus engineered attributes to optimize controlled protein expression. Attributes that make this a good cloning strain are the partially defective restriction systems (hsd and e14), a lack of the most common nuclease that copurifies with plasmid DNA isolated from E. coli strains (endA), and a preventive mutation to minimize undesirable recombination events (recA). KRX can be used for blue/white screening because it is deficient in β-galactosidase activity due to deletions in both genomic and episomal copies of the lacZ gene. The partial deletion in the episomal (F′ factor) copy of the lacZ gene (Δ(lacZ)M15) can be complemented by addition of a functional α-peptide encoded by a plasmid cloning vector. KRX also has attributes that make it a good protein expression strain. The ompT and ompP mutations eliminate one source of proteolysis of overexpressed protein in E. coli.