Tests of specific models of homologous recombination in Xenopus oocytes

DNA molecules recombine efficiently after injection into the nuclei of Xenopus laevis oocytes. Recombination in oocytes depends on sequence homology; no nonhomologous joining is detected. Double-strand breaks in DNA are very recombinogenic, implying the importance of molecular ends for efficient recombination. In the studies reported here, terminal nonhomologies at one or both ends inhibited recombination, and the degree of inhibition was related to the length of the nonhomology. However, the distribution of apparent crossovers was not significantly changed by a terminal nonhomology; apparent crossing over was highest near the ends of the homology, and quite low in the middle portion. This suggests a common intermediate in the recombination of nonblocked and blocked substrates; a full-length heteroduplex could be such an intermediate. Injection of a synthetic heteroduplex DNA showed the repair of mismatches with long excision tracts. The crossover point analysis also suggested that the resolution of recombination intermediates was the step that was inhibited by terminal nonhomologies. This hypothesis was confirmed by characterizing the structures of intermediates. Finally, a specific model of recombination, the double-strand-break repair model, was tested in Xenopus oocytes. This test was intended to distinguish whether recombination is conservative or nonconservative. The yield of double-strand-break repair recombination products was very low. These results indicate that the predominant recombination pathway in oocytes is nonconservative, but a low level of conservative recombination may also occur. Features of the nonconservative pathway are consistent with a single-strand annealing model of recombination.