DISTRIBUTION OF EXCHANGES UPON HOMOLOGOUS RECOMBINATION OF EXOGENOUS DNA IN XENOPUS-LAEVIS OOCYTES

Homologous recombination between DNA molecules injected into Xenopus o ocyte nuclei was investigated by examining the recovery of information from differentially marked parental sequences. The injected recombina tion substrate was a linear DNA with terminal direct repeats of 1246 b p; one repeat differed from the other by eight single base-pair substi tutions, distributed throughout the region of homology, each of which created or destroyed a restriction enzyme site. Recombination products were recovered and analyzed for their content of the diagnostic sites , either directly by Southern blot-hybridization or after cloning in b acteria. The majority (76%) of the cloned products appeared to be the result of simple exchanges-i.e., there was one sharp transition from s equences derived from one parent to sequences derived from the other. These simple exchanges were concentrated near the ends of the homologo us interval and, thus, near the sites of the original molecular ends. Placing marked sites on only one side of the homologous overlap showed that marker recovery was governed largely by the positions of the mol ecular ends and not by the markers themselves. When a terminal nonhomo logy was present at one end of the substrate, the yield of recombinant s was sharply decreased, but the pattern of exchanges was not affected , suggesting that products from end-blocked substrates arise by the sa me recombination pathway. Because of considerable evidence supporting a nonconservative, resection-annealing mechanism for recombination in oocytes, we interpret the distribution of exchanges as resulting from long-patch repair of extensive heteroduplex intermediates.