BL21DE3

Escherichia coli BL21(DE3) is a bacterial strain widely used in molecular biology for recombinant protein production. It is a derivative of the B strain BL21 and is optimized for high-level expression of proteins from plasmids that use the T7 RNA polymerase system. The DE3 designation refers to a lambda DE3 lysogen integrated into the chromosome, which carries the gene for T7 RNA polymerase under the lacUV5 promoter. When induced with IPTG, T7 RNA polymerase is produced and transcribes the target gene from a T7 promoter on the expression plasmid.

Key features include deficiency of major proteases Lon and OmpT, which reduces degradation of expressed proteins and improves yields. The strain is well suited to cytoplasmic expression of recombinant proteins and is commonly used with pET and related vectors. Its growth characteristics and tolerance to high expression levels have made it a workhorse in academic and industrial settings.

Limitations and considerations: basal expression can occur in the absence of inducer, and some proteins may be toxic to the host. Protein folding may require lower temperatures or co-expression of chaperones. For proteins requiring disulfide bonds, alternative strains or targeting to the periplasm may be preferable. Variants such as BL21(DE3) pLysS or pLysE provide additional regulation by expressing T7 lysozyme to suppress basal transcription.

See also: pET expression system; T7 expression; Studier.