AN IMPROVED SELECTION METHOD FOR LAMBDA-LACZ(-) PHAGES BASED ON GALACTOSE SENSITIVITY

The determination of the lacZ mutant frequency in lambda gt10lacZ phage vectors isolated from the transgenic mouse strain 40.6 (MutaMouse), requires the screening of large numbers of phages on beta-galactosidase activity. Existing methods rely on distinguishing a few white plaques on X-gal containing plates amongst a multitude of blue ones, which is both time-consuming and expensive. The new screening method described here employs the galactose sensitive Escherichia coli ClacZ recA galE strain into which a multicopy plasmid has been introduced, which results 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 without affecting the ability of lambda lacZ(-) phages to form plaques. With 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 been eliminated.