Roles of Hop1 and Mek1 in meiotic chromosome pairing and recombination partner choice in Schizosaccharomyces pombe.

Synaptonemal complex (SC) proteins Hop1 and Mek1 have been proposed to promote homologous recombination in meiosis of Saccharomyces cerevisiae by establishment of a barrier against sister chromatid recombination. Therefore, it is interesting to know whether the homologous proteins play a similar role in Schizosaccharomyces pombe. Unequal sister chromatid recombination (USCR) was found to be increased in hop1 and mek1 single and double deletion mutants in assays for intrachromosomal recombination (ICR). Meiotic intergenic (crossover) and intragenic (conversion) recombination between homologous chromosomes was reduced. Double-strand break (DSB) levels were also lowered. Notably, deletion of hop1 restored DSB repair in rad50S meiosis. This may indicate altered DSB repair kinetics in hop1 and mek1 deletion strains. A hypothesis is advanced proposing transient inhibition of DSB processing by Hop1 and Mek1 and thus providing more time for repair by interaction with the homologous chromosome. Loss of Hop1 and Mek1 would then result in faster repair and more interaction with the sister chromatid. Thus, in S. pombe meiosis, where an excess of sister Holliday junction over homologous Holliday junction formation has been demonstrated, Hop1 and Mek1 possibly enhance homolog interactions to ensure wild-type level of crossover formation rather than inhibiting sister chromatid interactions.