express tac promoter in BL21(DE3)

Kevin Brady Kevin.Brady at
Thu Jun 6 10:49:43 EST 2002

It's probably best you use just normal BL21 cells. The problem with T7 
RNA polymerase expression may be significant (although it may not) in 
BL21(DE3) as you will have 2 transcription suppressor proteins competing 
for the same amount of IPTG. JM109 is better for cloning steps, as it 
has fully active proteases (BL21 is lacking in the OmpT protease).

gianluca wrote:

> Hi, I read in a paper that a vector with Ptac promoter (pMal) was
> expressed in BL21(DE3) cells.
> Since I want to replicate this experiment, do you think that there
> could be problems with T7 RNA pol expression after IPTG addition?
> In case of affirmative answer, can you suggest a E. coli strain I
> could use? I thought of JM109.
> Thank you,
> Gianluca

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