FAQ: What is the difference between T7 Express (NEB #C2566H/I) and BL21(DE3) (NEB #C2527H/I)?

T7-Express has the T7 RNA polymerase gene inserted into the lac operon on the E. coli chromosome and is expressed under the control of the lac promoter. This configuration provides controlled induction of the polymerase and consequently, inducible control of transcription of genes downstream of the T7 promoter. This system provides potential advantages over strains such as BL21(DE3), that carry the T7 RNA polymerase on a lysogenic prophage. Although λDE3 is normally dormant in the host chromosome, the induction of the SOS cascade can occur as the result of expressing proteins that damage the E. coli chromosome, either directly or indirectly. This may lead to cell lysis.