insert-positive blue cols

Thomas J Radosevich tjrad at
Thu Jan 21 13:45:54 EST 1993

In <1993Jan21.090729.79 at> mspeek at (Mart Speek) writes:

>Hi Netters;

>I would like to know whether anyone has encountered problems
>in traditional blue/white colony or plaque screening procedures
>(beta galactosidase assay) when sorting out (non)recombinant
>clones. My question is :-( Could relatively long inserts (>0.5kb)
>sometimes give rise blue (beta gal+) clones? How is it possible?
>Any theoretical or practical solutions (incl refs) to this 
>particular problem are welcome.

>M. Speek

>mspeek at (ESTONIA)

I've had blue positives twice--one of 143 bp and the other 550 bp.  For the 
143 I unsuccessfully checked white colonies for quite some time.  Eventually
I checked blue colonies and they were all positive.  Now, whenever I clone a
small fragment, I always throw a few blue colonies in the first batch of
minipreps.  (BTW, these were bluescript/XL1 blue transformations)

Hope it helps--Tom Radosevich

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