A protein that holds DNA together is also needed to break it apart. Chad Ellermeier and Gerald Smith (Fred Hutchinson Cancer Research Center, Seattle, WA) show that the DNA glue cohesin regulates meiotic double-strand break (DSB) formation and recombination in fission yeast.
The cohesins that hold sister chromatids together during meiosis, called Rec8 and Rec11, are early arrivals on chromosomes during premeiotic replication. They provide a binding site for Rec10, which is a main component of linear elements—fission yeast's version of synaptonemal complexes.
Ellermeier and Smith found that deletion of Rec8 and Rec11 caused region-specific decreases in DSB formation and recombination. Loss of Rec10 blocked breakage and recombination throughout the genome. The team thinks the widespread problems in Rec10 occur because it must be present to bring in the enzyme that actually clips the DNA, called Rec12. Rec8 and Rec11, by contrast, are not evenly distributed over the meiotic chromosomes, and thus their absence only causes intermittent problems.
“It is surprising that cohesins, which hold sister chromatids together, are so important in recombination,” says Smith, “because cross-overs occur between homologues.” But the ordered loading process explains the puzzle. Cohesins are the first to load onto the meiotic chromosomes and must be there for the rest of the events to follow.