High sensitivity for color mutants in lacZ plasmid-based transgenic mice, as detected by positive selection

Transgenic mice carrying bacteriophage lambda-based vectors harboring a lacZ or lacI reporter gene have been used in recent years for the detection, quantification, and characterization of spontaneous and induced gene mutations in vivo. The usefulness of these models is basically determined by their ability to detect significant (organ-specific) exposure-related increases in mutant frequencies. This, in its turn, is dependent on the models' ability to detect all possible types of mutations, including mutations that partially inactivate the gene product, as well as the absence of a significant proportion of E. coli-derived mutations. The newly developed lacZ plasmid-based transgenic mouse mutation assay is sensitive to a broad range of DNA mutations, including large size changes. Here, we describe the plasmid model's sensitivity for detecting color mutants, i.e., cells containing a partially inactivated lacZ gene, as detected by a positive selection system using phenyl-beta-galactoside. The proportion of color mutants in brain and kidney was 30-40%. This was significantly higher than the 10-20% color mutants observed in lung, spleen, and liver. It is demonstrated that the color mutants detected were neither the result of a mixture of both mutant and nonmutant cells within the original mutant colony-forming unit nor due to the presence of mutant and nonmutant lacZ genes within individual E. coli host cells. Sequence analysis of 13 different color mutants revealed single basepair substitutions in the lacZ gene of each mutant. The high tolerance of the positive selection system for lacZ color mutants in the plasmid-based transgenic mouse model greatly contributes to the sensitivity of this model, now ranging from large size-change mutations that completely inactivate the lacZ gene to basepair substitutions that partly inactivate the lacZ gene.