E. coli XL1-Blue MR strain - question!

Michael Benedik benedik at uh.edu
Tue Apr 16 16:56:27 EST 1996

In article <dkearns-1604961353490001 at shimkets_si.mib.uga.edu>
dkearns at uga.cc.uga.edu (Dan Kearns) writes:

>   I am having trouble getting a straight answer from stratagene.  I want
> to know if this strain has a functional lacY gene.  I realize that it is
> most certainly LacZ-.  However if lacZ is expressed from a plasmid, will
> the cells grow on mm+lac media??  The genotype is the same as the more
> common XL1 Blue MRF', except the absence of the F' lacks alpha
> complementation, and Tet.  They say the lac mutation is uncharachterized
> and are not sure if the strain is lacY + or -.  If any one can enlighten
> me I would be very greatful.

It should be lacY+, that F' is supposed to carry the entire lac operon
(plus other stuff) and just have the M15 deletion internal to the lacZ

If you put in a plasmid to get complementation, it will probably not
grow very well on lac minimal. The complementation between the alpha of
the plasmid and the lacZM15 is not very efficient and if you measure
lacZ activity it will be quite low. But XGal is very sensitive and you
will get a blue colony at very very low levels of lacZ activity which
would not otherwise support good growth on lactose.

I am not sure if it won't grow at all, or just very slowly, but it
certainly is very different than wild type lacZ+

Michael Benedik
Department of Biochemical Sciences
University of Houston
benedik at uh.edu

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