Cannavo E, Sanchez A, Anand R, Ranjha L, Hugener J, Adam C, Acharya A, Weyland N, Aran-Guiu X, Charbonnier JB, Hoffmann ER, Borde V, Matos J, Cejka P

During prophase of the first meiotic division, cells deliberately break their DNA1. These DNA breaks are repaired by homologous recombination, which facilitates proper chromosome segregation and enables the reciprocal exchange of DNA segments between homologous chromosomes2. A pathway that depends on the MLH1-MLH3 (MutL?) nuclease has been implicated in the ...

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biased processing of meiotic recombination intermediates into crossovers by an unknown mechanism3-7. Here we have biochemically reconstituted key elements of this pro-crossover pathway. We show that human MSH4-MSH5 (MutS?), which supports crossing over8, binds branched recombination intermediates and associates with MutL?, stabilizing the ensemble at joint molecule structures and adjacent double-stranded DNA. MutS? directly stimulates DNA cleavage by the MutL? endonuclease. MutL? activity is further stimulated by EXO1, but only when MutS? is present. Replication factor C (RFC) and the proliferating cell nuclear antigen (PCNA) are additional components of the nuclease ensemble, thereby triggering crossing-over. Saccharomyces cerevisiae strains in which MutL? cannot interact with PCNA present defects in forming crossovers. Finally, the MutL?-MutS?-EXO1-RFC-PCNA nuclease ensemble preferentially cleaves DNA with Holliday junctions, but shows no canonical resolvase activity. Instead, it probably processes meiotic recombination intermediates by nicking double-stranded DNA adjacent to the junction points9. As DNA nicking by MutL? depends on its co-factors, the asymmetric distribution of MutS? and RFC-PCNA on meiotic recombination intermediates may drive biased DNA cleavage. This mode of MutL? nuclease activation might explain crossover-specific processing of Holliday junctions or their precursors in meiotic chromosomes4.

Date: Dec. 01, 2019

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