Direct selection for GalE in E.coli

Duncan Clark Duncan at
Tue Mar 21 06:29:31 EST 1995

In article: <Pine.SOL.3.91.950320120137.26201A-100000 at corona>  Rafael Maldonado <rafael at> writes:
> On Mon, 20 Mar 1995, Duncan Clark wrote:
> [edited] 
> > Why do I want a GalE delta(lac-proAB) host? If you use pUC18 in such 
> > a host with IPTG/Xgal etc the alpha complementation is lethal with
> > phenylgalactoside. This gives a direct selection for whites. Just 
> > think, no phosphatasing and everything has an insert.
> HA! You're not the first one in thinking of that.
> Gossen et al. (1992) Application of galactose-sensitive E. coli strains as 
> selective hosts for LacZ- plasmids. NAR 20, 3254.
> I think that you can get some other mutation that mimics the toxicity of 
> galactose, but I think happens at very low frequency. Others mutations 
> may be in the lacZ of your plasmid.
> Why are not people using this system? You know, MBist are too lazy to 
> change techniques... and the X-gal system works pretty well.

Unfortunatley this strain does not give alpha complementation (regardless of 
what the paper says) so will not give a selection with any pUC or similar

My mind's made up. Don't confuse me with the facts!

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