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

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.