Background mutations and polymorphisms in lacZ-plasmid transgenic mice

Transgenic animal models harboring chromosomally integrated shuttle vectors with bacterial reporter genes are now widely used to measure in vivo mutan t Frequencies. The lacZ-plasmid transgenic mouse model has a unique sensiti vity to large rearrangements compared to systems using bacteriophage lambda vectors, which mainly detect point mutations and small deletions or insert ions. In this study, the background mutant frequencies and spectra in the l aci-plasmid transgenic mouse model were investigated. While the majority of the recovered lacZ-mutants appeared to have originated in the mouse, a sub set of mutants are likely to represent artifacts, and occur with a frequenc y of about 1.3 x 10(-5), irrespective of the total mutant frequency. Galact ose-insensitive host cells, due to galE back mutations or galK or galT Forw ard mutations, grow through the positive selection system and cause a smell subset of the background. When using HindIII to excise the plasmids From g enomic DNA, the largest contribution to the background, (1.1 +/- 0.3) x 10( -5), appeared to be caused by star activity, i.e., cleavage at nucleotide s equences other than the HindIII restriction enzyme recognition sequence, du ring the recovery procedure. Finally, a total of 10 polymorphic sites in di fferent copies of the lacZ-plasmid cluster in founder line 60 were discover ed. (C) 1999 Wiiey-Liss, Inc.