Toxic beta-galactosidase substrate?

[Dr. Hedley Carr]

Does anyone know of a beta-galactosidase substrate that yields
a product toxic to E.coli only if it is cleaved ? (i.e: only if 
the cells are expressing LacZ). I'm after a selection method which
will screen out all unwanted non-recombinants (i.e: LacZ intact).

Thanks.

Hedley Carr.

School of Biochem. & Mol. Biol.,
University of Leeds.

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