GENETIC-RECOMBINATION THROUGH DOUBLE-STRAND BREAK REPAIR - SHIFT FROM2-PROGENY MODE TO ONE-PROGENY MODE BY HETEROLOGOUS INSERTS

Double-strand break repair models of genetic recombination propose tha t a double-strand break is introduced into an otherwise intact DNA and that the break is then repaired by copying a homologous DNA segment. Evidence for these models has been found among lambdoid phages and dur ing yeast meiosis. In an earlier report, we demonstrated such repair o f a preformed double-strand break by the Escherichia coli RecE path wa y. Here, our experiments with plasmids demonstrate that such reciproca l or conservative recombination (two parental DNAs resulting in two pr ogeny DNAs) is frequent at a double-strand break even when there exist s the alternative route of nonreciprocal or nonconservative recombinat ion (two parental DNAs resulting in only one progeny DNA). The presenc e of a long heterologous DNA at the double-strand break, however, resu lted in a shift from the conservative (two-progeny) mode to the noncon servative (one-progeny) mode. The product is a DNA free from the heter ologous insert containing recombinant flanking sequences. The potentia l ability of the homology-dependent double-strand break repair reactio n to detect and eliminate heterologous inserts may have contributed to the evolution of homologous recombination, meiosis and sexual reprodu ction.