The determination of the lacZ mutant frequency in lambda gt10lacZ phag
e vectors isolated from the transgenic mouse strain 40.6 (MutaMouse),
requires the screening of large numbers of phages on beta-galactosidas
e activity. Existing methods rely on distinguishing a few white plaque
s on X-gal containing plates amongst a multitude of blue ones, which i
s both time-consuming and expensive. The new screening method describe
d here employs the galactose sensitive Escherichia coli ClacZ recA gal
E strain into which a multicopy plasmid has been introduced, which res
ults in over-expression of the galK and galT genes. In the presence of
phenyl-beta-D-galactopyranoside, a substrate for beta-galactosidase,
this leads to the suppression of lambda lacZ(+) phage propagation with
out affecting the ability of lambda lacZ(-) phages to form plaques. Wi
th this method it is possible to screen 1.5 x 10(6) phages on a single
9-cm Petri dish. Furthermore, the need for blue/white screening has b
een eliminated.