COMPARISON OF THE MOLECULAR CHARACTERISTICS OF LACZ TRANSGENIC MOUSE MUTATIONS DETECTED BY VISUAL AND POSITIVE SELECTION

lacZ gene mutations in the transgenic Muta(TM)Mmouse can be detected b y two different selection systems. While mutant frequencies recovered by phenyl-beta-D-galactoside (P-gal) selection are comparable with tho se obtained using 5-bromo-4-chloro-3-indolyl beta-D-galactopyranoside (X-gal) as substrate for beta-galactosidase, there may still be differ ences at the molecular level in the mutations detected by these two me thods. Accordingly, we have examined a spectrum of mutants recovered f rom the X-gal system by regrowing these mutants on the P-gal plates. A ll colourless X-gal mutants grew normally on the P-gal plates. However , 11 out of 53 single light blue mutants, which express partial beta-g alactosidase activity, produce few or no plaques on the P-gal plates, indicating the possible loss of some mutations using the positive sele ction system. Further analysis of mutant phenotypes and base changes i ndicates that such loss is not mutation-type specific. Our data sugges t that the positive selection method can detect the majority of lacZ m utations detectable by visual selection and is more efficient at detec ting mutants within a reduced range of beta-galactosidase activity.