Origami B host strains carry the same mutations in trxB and gor as the original Origami strains, except that they are derived from a lacZY mutant of BL21 to enable precise control of expression levels by adjusting the concentration of IPTG. Thus the Origami B strains combine the desirable characteristics of BL21, Tuner?, and Origami strains in one strain background. The mutations in trxB and gor are selectable on kanamycin and tetracycline, respectively; therefore, these strains cannot be used with plasmids that can only selected with kanamycin or tetracycline. These strains also include the lon and ompT deficiences of BL21, which increase protein stability. DE3 indicates that the host is a lysogen of λDE3, and therefore carries a chromosomal copy of the T7 RNA polymerase gene under control of the lacUV5 promoter. Such strains are suitable for production of protein from target genes cloned in pET vectors by induction with IPTG. pLysS strains express T7 lysozyme, which further suppresses basal expression of T7 RNA polymerase prior to induction, thus stabilizing pET recombinants encoding target proteins that affect cell growth and viability.
