Blue and white

Michael Benedik benedik at
Wed Oct 2 16:03:06 EST 1996

In article <52tkfl$ndd at>
"FABIAN MOEBIUS" <Fabian.Moebius at> writes:

> Hello,
> It takes my blue/white screen more than 48 hours to become blue (pBS 
> in XL-1 Blue), but after a long while the colonies  become deep blue. We spread 100  l of 100 mM IPTG and 40  l of Bluo-Gal 
> (Sigma, 40 mg/ml) per 10 cm dish. Which factors influence color 
> development? Is it critical to dissolve X-gal in dimethylformamide?
> Thanks
> Dr. Fabian F. MOEBIUS
> Institut fuer Biochemische Pharmakologie
> Peter Mayr Str. 1
> 6020 Innsbruck
> TEL ++43-512-507-3155
> FAX ++43-512-588627

XGAL is the agent that actually turns blue. IPTG just induces
transcription from the lac promoter.  Xgal is not soluble in aqueous
solution (at least not at 
40mg/ml, so you need to dissolve it in DMF or DMSO.

You should be able to see blue color after overnight growth. Try making
your solutions up again fresh. Also be sure your XL-1 blue strain still
carries the 
F' episome (Tet-R).

Michael Benedik
Department of Biochemical Sciences
University of Houston
benedik at

More information about the Methods mailing list