DE3

DE3 is a genotype designation used for Escherichia coli strains that harbor a lambda DE3 prophage integrated into the chromosome. The prophage carries the gene for T7 RNA polymerase under control of the lacUV5 promoter. When cells are exposed to IPTG, the LacI repressor is inactivated and T7 RNA polymerase is produced. The enzyme then recognizes promoters of the T7 type on expression plasmids and drives high-level transcription of the downstream gene, enabling efficient production of recombinant proteins.

DE3-containing strains are most often used with plasmids that carry a gene under a T7 promoter, such as those in the pET expression system. Common host backgrounds include BL21 (DE3), BL21 derivatives carrying codon-usage supplements (e.g., Rosetta(DE3)), and other B- or K-12–derived strains. In many cases, additional features are present, such as mutations that reduce host protease activity or auxiliary plasmids that modulate expression levels to balance yield and protein quality.

Practical use typically involves transforming the expression plasmid into a DE3 host, growing to mid-log phase, and inducing with IPTG to initiate transcription. High expression levels can lead to inclusion bodies or misfolded proteins, so conditions are often optimized by lowering temperature, reducing inducer concentration, or reducing induction duration. Some strains carry pLysS or pLysE plasmids to suppress unwanted basal expression and improve protein solubility. Other variants, such as strains with enhanced rare-codon tRNA supplies, expand the range of proteins that can be expressed efficiently.

Safety and handling follow standard laboratory practices for Escherichia coli strains, typically classified as BSL-1 organisms. DE3 remains a foundational tool in molecular biology for rapid, controllable protein production.