|Genotype||F(-) ompT gal dcm lon hsdSB(rB(-) mB(-)) λ(DE3 [lacI lacUV5-T7 gene 1 ind1 sam7 nin5])|
The strain contains DE3, a λ prophage carrying the T7 RNA polymerase gene under control of the lac UV5 promoter and lacIq. IPTG is required to induce expression of the T7 RNA polymerase. Protein expression from transforming plasmids containing a T7 promoter-driven expression system is repressed until IPTG induction of T7 RNA polymerase expression occurs. The strain does not contain the lon protease and is also deficient in the outer membrane protease, OmpT. The lack of two key proteases reduces the degradation of heterologous proteins expressed in the strain. The helper plasmid pLysE produces higher amounts of T7 lysozyme than pLysS to further reduce basal level expression of recombinant genes by inhibiting basal levels of T7 RNA polymerase. This makes BL21(DE3)[pLysE] more suitable for expressing toxic genes. pLysE also carries the chloramphenicol resistance gene and the p15A origin for compatibility with plasmids containing the ColE1 or pMB1 origin.
The full genome sequence of the BL21(DE3) strain is available at the ENA/GenBank nucleotide sequence databases under accession number CP001509.3.