blue colonies-no Xgal

Rafael Maldonado rafael at
Mon Jul 1 18:37:07 EST 1996

On Thu, 27 Jun 1996, Michael H. Bracey wrote:

> I recently cloned some PCR products into InVitrogen's tailed vector and 
> transformed the ligation products into their SureShot Top10 cells which 
> are supposed to have a lacIq repressor.  In the *absence* of both IPTG 
> and X-gal, I got back about 50% blue colonies.  Is there a trivial 
> explanation for this that I'm overlooking?  Where is the blue coming 
> from?


If you did not add X-gal to your plates nor broth, the only explanation 
is that X-gal came with those suretop competent cells!! Washing the cells 
before plating may give you an answer.

Check the supplier technical assistance; maybe they add X-gal to their 
competent cells so you do not need to add it to the plate. If the 
promoter is tac, you might get blue color even in presence of lacIq and 
absence of IPTG.


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