- The λDE3 lysogen is a genetically engineered element derived from bacteriophage λ, stably integrated into the chromosome of certain Escherichia coli (E. coli) strains, such as BL21(DE3).
- This lysogen carries the gene encoding T7 RNA polymerase, which is essential for the T7 promoter-based expression system.
- Importantly, the expression of T7 RNA polymerase is placed under the control of the lacUV5 promoter, a strong and IPTG-inducible variant of the lac promoter that is less sensitive to catabolite repression and provides tighter control over induction.
- When IPTG is added to a culture of BL21(DE3) or a similar strain, it inactivates the Lac repressor, thereby inducing the lacUV5 promoter and leading to the production of T7 RNA polymerase. This enzyme, in turn, specifically recognizes and transcribes genes located downstream of T7 promoters—commonly found in pET plasmids—at very high efficiency. The T7 RNA polymerase is highly processive and selective, which allows for strong and rapid expression of the target gene without significant interference from host RNA polymerases.
- The presence of the λDE3 lysogen thus forms the backbone of the widely used T7 expression system, making it a powerful tool in molecular biology and biotechnology.
- However, this strong expression can sometimes be a double-edged sword; it may lead to protein misfolding, formation of inclusion bodies, or cell toxicity if not carefully regulated. For such cases, derivative strains like BL21(DE3)pLysS are used to reduce basal expression.
- Nevertheless, the λDE3 lysogen remains central to high-yield recombinant protein production in E. coli and has played a major role in enabling the widespread use of bacterial systems for producing research proteins, enzymes, and biopharmaceuticals.
