Inhibition of the Smc5/6 Complex during Meiosis Perturbs Joint Molecule Formation and Resolution without Significantly Changing Crossover or Non-crossover Levels

Abstract

Meiosis is a specialized cell division used by diploid organisms to form haploid gametes for sexual reproduction. Central to this reductive division is repair of endogenous DNA double-strand breaks (DSBs) induced by the meiosis-specific enzyme Spo11. These DSBs are repaired in a process called homologous recombination using the sister chromatid or the homologous chromosome as a repair template, with the homolog being the preferred substrate during meiosis. Specific products of inter-homolog recombination, called crossovers, are essential for proper homolog segregation at the first meiotic nuclear division in budding yeast and mice. This study identifies an essential role for the conserved Structural Maintenance of Chromosomes (SMC) 5/6 protein complex during meiotic recombination in budding yeast. Meiosis-specific smc5/6 mutants experience a block in DNA segregation without hindering meiotic progression. Establishment and removal of meiotic sister chromatid cohesin are independent of functional Smc6 protein. smc6 mutants also have normal levels of DSB formation and repair. Eliminating DSBs rescues the segregation block in smc5/6 mutants, suggesting that the complex has a function during meiotic recombination. Accordingly, smc6 mutants accumulate high levels of recombination intermediates in the form of joint molecules. Many of these joint molecules are formed between sister chromatids, which is not normally observed in wild-type cells. The normal formation of crossovers in smc6 mutants supports the notion that mainly inter-sister joint molecule resolution is impaired. In addition, return-to-function studies indicate that the Smc5/6 complex performs its most important functions during joint molecule resolution without influencing crossover formation. These results suggest that the Smc5/6 complex aids primarily in the resolution of joint molecules formed outside of canonical inter-homolog pathways.

SMC5/6 is required for the formation of segregation-competent bivalent chromosomes during meiosis I in mouse oocytes

“…SMC5/6 was also visible along chromosome axes at pachynema in oocytes, which was also detected in mouse spermatocytes (Gomez et al, 2013). In addition, transient foci of SMC6 were detected along the chromosome arms in female germ cells during pachynema, suggesting a role during meiotic recombination, which has previously been reported using budding yeast and Caenorhabditis elegans (Bickel et al, 2010;Hong et al, 2016;Checchi et al, 2014;Copsey et al, 2013;Lilienthal et al, 2013;Xaver et al, 2013). In mammals, every chromosome pair obtains many recombination sites but generally yields only one to two crossover sites (Kauppi et al, 2004).…”

“…Contrasting data have been reported on whether mutation of cohesin component, REC8, affects Smc5/6 axis loading during meiosis in budding yeast (Copsey et al, 2013;Lilienthal et al, 2013). Localization of SMC6 was assessed using a Rec8 mouse mutant (Bannister et al, 2004).…”

Selective modulation of the functions of a conserved DNA motor by a histone fold complex

“…S4A-C). We note that, similar to scMHF, Smc5/6 does not affect CO:NCO control (Lilienthal et al 2013; S Gangloff, pers. comm.)…”

Meiotic Recombination: The Essence of Heredity

“…Similar to cells lacking STR function, mutation of the Smc5/6 complex increases the levels of intersister and multichromatid joint molecules and the expense of interhomolog dHJs (Copsey et al 2013;Lilienthal et al 2013;Xaver et al 2013). Analysis in both budding and fission yeasts shows that additional aberrant joint-molecule structures arise when Smc5/6 is defective (Wehrkamp-Richter et al 2012;Copsey et al 2013).…”