FAQ: What is the difference between BL21 and BL21(DE3) competent E.coli?

Both strains are B strains and thus both are deficient in Lon protease (cytoplasm) and OmpT protease (outer membrane). Accordingly, B strains are generally preferred for recombinant protein expression. The DE3 designation means that respective strains contain the λDE3 lysogen that carries the gene for T7 RNA polymerase under control of the lacUV5 promoter. IPTG is required to maximally induce expression of the T7 RNA polymerase in order to express recombinant genes cloned downstream of a T7 promoter.

BL21(DE3) is suitable for expression from a T7 or T7-lac promoter or promoters recognized by the E.coli RNA polymerase: e.g. lac, tac, trc, ParaBAD, PrhaBAD and also the T5 promoter.

Note that BL21 does not carry the gene for T7 RNA polymerase and thus is only suitable for expression from promoters recognized by the E.coli RNA polymerase: e.g. lac, tac, trc, ParaBAD, PrhaBAD and also the T5 promoter.