BL21(DE3) and trc promoter

[Dr. Peter Gegenheimer]

On Tue, 8 Feb 2000 12:29:35, Petri Kursula <pkursula at cc.oulu.fi> wrote:

ð Confused by all these E.coli genotypes and strains, I would like to know
ð if anyone has used BL21(DE3) cells to produce recombinant proteins from a
ð vector with the trc promoter, such as pTrc99a. If yes, is this expression
ð tightly regulatable by IPTG, or do you get leaky expression?

You certainly don't want the (DE3) -- this is the phage T7 RNA polymerase gene
under lac control. BL21(DE3) cells cannot be transformed by any expression 
plasmid which has a lac operator on it, unless the plasmid also has the lacIq 
supre-repressor. Even from a plasmid carrying lacIq, you certainly don't need 
to be co-expressing T7 RNAP! 

Try plain BL21 (remember, it's an E. coli B strain) or another 
protease-deficient strain if that's your goal. 

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