Comparison of the molecular characteristics of lacZ transgenic mouse mutations detected by visual and positive selection.

lacZ gene mutations in the transgenic Muta Mmouse can be detected by two different selection systems. While mutant frequencies recovered by phenyl-beta-D-galactoside (P-gal) selection are comparable with those obtained using 5-bromo-4-chloro-3-indolyl beta-D-galactopyranoside (X-gal) as substrate for beta-galactosidase, there may still be differences at the molecular level in the mutations detected by these two methods. Accordingly, we have examined a spectrum of mutants recovered from the X-gal system by regrowing these mutants on the P-gal plates. All colourless X-gal mutants grew normally on the P-gal plates. However, 11 out of 53 single light blue mutants, which express partial beta-galactosidase activity, produce few or no plaques on the P-gal plates, indicating the possible loss of some mutations using the positive selection system. Further analysis of mutant phenotypes and base changes indicates that such loss is not mutation-type specific. Our data suggest that the positive selection method can detect the majority of lacZ mutations detectable by visual selection and is more efficient at detecting mutants within a reduced range of beta-galactosidase activity.