- Escherichia coli (E. coli) BL21(DE3) is one of the most widely used bacterial strains for recombinant protein expression due to its combination of high yield, ease of use, and compatibility with powerful expression systems.
- It is derived from the BL21 strain, a protease-deficient E. coli B lineage that lacks the lon and ompT proteases, which helps reduce degradation of heterologous proteins during and after expression.
- The defining feature of BL21(DE3) is the presence of the λDE3 lysogen, which harbors the gene for T7 RNA polymerase under the control of the lacUV5 promoter. This setup enables strong, IPTG-inducible expression of target genes placed downstream of a T7 promoter, such as in pET expression vectors.
- Upon induction with IPTG, the host produces T7 RNA polymerase, which drives high-level transcription of the target gene with high specificity and speed. This system is highly efficient and suitable for the production of a wide range of recombinant proteins, particularly when high expression levels are desired. However, due to the powerful nature of the T7 promoter system, proteins may accumulate rapidly and lead to the formation of inclusion bodies, especially for eukaryotic proteins or those requiring complex folding.
- BL21(DE3) is typically the starting strain for many protein expression experiments and forms the basis for several enhanced derivatives, such as BL21(DE3)pLysS (for reduced basal expression) and BL21(DE3) CodonPlus (for improved expression of rare-codon-rich genes). While it does not perform post-translational modifications, and its expression may need optimization for solubility or toxicity issues, E. coli BL21(DE3) remains a workhorse in molecular biology due to its speed, simplicity, and high productivity.
