FIG. 3.

An unstable insertion mutant. (A) The freezer stock of the unstable insertion mutant (the yncD yddW mutant) was streaked onto an LB plate containing X-Gal (60 μg/ml) and kanamycin (60 μg/ml). A dark-blue and a light-blue colony were picked and restreaked onto a second plate of the same composition, along with a new streak from the same freezer stock (original mutant). The plates were incubated overnight at 37°C and photographed the next morning. The light-blue color is very similar to that of parental strain, JH39 (i.e., SOS repressed). (B) The gene organization of the wild-type terminal region is depicted, with emphasis on the sites involved in the transposition events. PCR primers used in the DNA analysis are indicated above the relevant reading frames, which are color coded (shades of blue, green, and yellow). A large grey arrow indicates the orientation of the intervening 47-kb chromosomal segment. (C) The inferred genome arrangement of the inversion mutant, prior to excisive recombination, is depicted above a downward arrow. Homologous recombination between the directly repeated transposon insertions (indicated in red) can excise a circle containing one copy of the transposon and a segment of the terminus region (diagram below +). Loss of the excised circle during cell division is proposed to explain the generation of colonies that are SOS repressed but contain a large deletion of terminus DNA.