GIN-MEDIATED RECOMBINATION AT SECONDARY CROSSOVER SITES ON THE ESCHERICHIA-COLI CHROMOSOME

The Cin recombinase is known to mediate DNA inversion between two wild -type cix sites flanking genetic determinants for the host range of ba cteriophage P1. Cin can also act with low frequency at secondary (or q uasi) sites (designated cixQ) that have lower homology to either wild- type site. An inversion tester sequence able to reveal novel operon fu sions was integrated into the Escherichia coli chromosome, and the Cin recombinase was provided in trans. Among a total of 13 Cin-mediated i nversions studied, three different cixQ sites had been used. In two re arranged chromosomes, the breakpoints of the inversions were mapped to cixQ sites in supB and ompA, representing inversions of 109 and 210 k b, respectively. In the third case, a 2.1-kb inversion was identified at a cixQ site within the integrated sequences. This derivative itself was a substrate for a second inversion of 1.5 kb between the remainin g wild-type cix and still another cixQ site, thus resembling a reversi on. In analogy to that which is known from DNA inversion on plasmids, homology of secondary cix sites to wild-type recombination sites is no t a strict requirement for inversion to occur on the chromosome. The c hromosomal rearrangements which resulted from these Cin-mediated inver sions were quite stable and suffered no growth disadvantage compared w ith the noninverted parental strain. The mechanistic implications and evolutionary relevance of these findings are discussed.