Mutagenesis, Vol. 14, No. 3, 287-293,
May 1999
© 1999 UK Environmental Mutagen Society/Oxford University Press
Characterization of color mutants in lacZ plasmid-based transgenic mice, as detected by positive selection
1 Beth Israel Deaconess Medical Center and Harvard Medical School, Harvard Institutes of Medicine, 77 Avenue Louis Pasteur, Boston, MA 02115, USA and 2 Novartis Pharma AG, Toxicology/Pathology, Genetic Toxicology, CH-4002 Basel, Switzerland
The plasmid-based transgenic mouse model, which uses the lacZ gene as the target for mutation, is sensitive to a wide range of in vivo mutations, ranging from point mutations to insertions and deletions extending far into the mouse genome. In this study, the nature of subtle lacZ mutations, which do not completely abolish ß-galactosidase activity, as detected by positive selection, was investigated. These subtle mutants are called `color mutants' due to their light blue staining on X-gal medium. Replating of color mutants and retransformation of plasmid DNA, purified from individual color mutants, resulted in the same phenotype as the original color mutant. The p-gal positive selection system tolerates ~10% of wild-type activity as indicated by spectrophotometric determination of ß-galactosidase activity of individual color mutants. Restriction digestion and size separation of plasmid DNA revealed no visible change in the size of the plasmid in color mutants. Sequence analysis confirmed the presence of a point mutation in each lacZ gene of nine different color mutants. The results indicate that color mutants are caused neither by the presence of a mixture of wild-type and mutated lacZ plasmids within the same host cell nor by a mixture of cells within the original mutant colony which carry either wild-type or mutated lacZ plasmids. In addition, it was discovered that the mouse line studied harbors four polymorphic base changes among the integrated plasmid copies.
3 Present address: Institute for Drug Development, Cancer Therapy and Research Center, 8122 Datapoint Drive, Suite 700, San Antonio, TX 78229, USA
4 To whom correspondence should be addressed. Tel: +1 210 616 5850; Fax: +1 210 692 7502; Email: mdolle{at}saci.org
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