| A branched DNA recombination intermediate (joint molecules, JM) has been isolated and physically characterized from meiotic cultures of the yeast Saccharomyces cerevisiae. JMs have the following characteristics: (1) Their occurrence is meiosis-specific, (2) Their abundance correlates with loci active in meiotic recombination and meiosis-specific DNA double strand break (DSB) formation, (3) They are formed after DSBs and prior to mature recombinant DNA duplexes, (4) Mutants that specifically block meiotic recombination coordinately block formation of JMs, (5) JMs contain full length DNA strands from bath parental homologs, (6) Approximately a quarter of these component strands are recombinant for an internal region, (7) JMs can be quantitatively resolved in vitro into both parental and recombinant duplexes using nucleases that specifically resolve Holliday junctions. From these observations, I conclude that JMs represent bona fide recombination intermediates. Further structural considerations indicate that JMs represent double Holliday junctions, an intermediate previously proposed in the repair of DSBs.;JMs form predominately between the two parental homologs, with only a small fraction forming between sister chromatids. To search for the genetic basis of this meiotic preference toward interhomolog recombination, JM formation was assayed in previously identified meiotic recombination mutants. Sequence homologs to the E. coli recA protein, and the meiotic chromatin structure protein RED1 were analyzed. These studies provide strong evidence that recombination between sister chromatids and homologs are differentially regulated, probably at a point prior to any physical change in the DNA. Meiosis-specific components RED1 and the meiosis-specific recA homolog DMC1 alter the normal mitotic preference for intersister recombination to the interhomolog mode favored during meiosis. The mitotic recA homologs RAD51, 55 and 57 function together during mitotic recombination, and are likely to function as the "core" of the recombination machinery. |
