BL21 question

Michael Benedik benedik at
Mon Apr 27 09:08:10 EST 1998

In article <1998Apr27.132035.32577 at>
chen at (Chen Ho An) writes:

> Does anyone know from which strain is BL21 derived from?  I'm asking this
> because the Health & Safety people here thinks BL21 is unsafe because it
> is not a K12 or B strain derivative and do not have sufficient evidence
> of its disablement (don't know what that means or refer to - probably
> refer to the transmissibility of the plasmids).  We may not therefore use
> it in the future.  I've always thought that BL21 is a B strain derivative
> but it seems I'm wrong, so, what is it derived from?
> p.s  does anyone knows of other good expression strains?
> --

BL21 is an E. coli B strain (but not K12). I think the original
reference if you want to prove it to them is

Studier and Moffet 1986 JMB 189:113-130
or Studier et al 1990 Methods in Enzymology 185:60-89

or you can show them the E. coli strain genotype table in various
catalogs like New England Biolabs (which explicitly states it is an E.
coli B strain)
or Novagen catalog.

Novagen also sells some K12 strains which express T7 polymerase
like NovaBlue(DE3) or HMS173(DE3). I think the HMS strain also comes
from Studier, but the NovaBlue strain is theirs.

They also sell a kit so you can make a DE3 lysogen of your favorite K12
strain if you don't have the phages to do it yourself.

Michael Benedik
Department of Biochemical Sciences
University of Houston
benedik at

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