BL21 expression host

Dima Klenchin klenchin at
Wed Feb 7 15:19:05 EST 2001

DOCTORHIM at wrote:
:The expression of T7 polymerase in BL21(DE3) is very leaky.  Many clones 
:don't even require induction for high level expression.  Glucose will not 
:help,  as the promoter for the polymerase is lacUV5 which is a catabolite 
:insensitive promoter.  This is probably the reason it is so leaky. 

True but I would put it conditionally - "can be very leaky". With some 
proteins I don't even need to add IPTG to get optimal expression, while
with others it seems to be tight enough so as I do not see the major 
band by coomassie staining. I don't understand where the difference
is coming from.

Using DE3 with pLys S or E usually helps but frequently at expense
of expression levels. 

        - Dima

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