Roles of Hop1 and Mek1 in Meiotic Chromosome Pairing and Recombination Partner Choice in Schizosaccharomyces pombe

Abstract

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.Sexual reproduction in eukaryotes involves formation of haploid gametes from diploid cells by one round of DNA replication, pairing of the homologous chromosomes, and recombination and then by the two meiotic divisions (53). In fungi the gametes differentiate into haploid spores, which germinate to form vegetative cells. Crossover (CO) formation between homologous chromosomes and DNA repair processes between sister chromatids are required for spore viability (10,55,58).In vegetative cells homologous recombination (HR) is important for repair of DNA damage and stalled replication forks, with the sister chromatid as the preferred partner (28). Many of the enzymes involved in mitotic HR also contribute to meiotic recombination. In addition, meiosis-specific cytological structures and enzymes enhance recombination frequency (meiotic induction) and shift partner preference from sister chromatids to homologous chromosomes (3,47,64,74). In detail the steps of HR vary between different types of sequence organization (allelic versus sister versus ectopic), between different types of DNA damage, between meiotic and mitotic cells, and between species (10, 55, 58).Meiotic recombination, including CO formation, is initiated by DNA double-strand breaks (DSBs). In Saccharomyces cerevisiae and other eukaryotes, DSBs are formed by Spo11. Many cofactors are required (29). The Schizosaccharomyces pombe homolog is Rec12, also requiring auxiliary factors whose elimination leads to loss of meiotic DSB formation (12). The 5Ј single-strand ends at DSBs are processed by nucleases. In S. cerevisiae the MRX complex made up by the proteins Rad50, Mre11, and Xrs2 is req...

The Recombinases DMC1 and RAD51 Are Functionally and Spatially Separated during Meiosis in Arabidopsis

“…We furthermore revealed that DMC1-coated nucleoprotein filaments are in principle competent to repair from both sister chromatids and homologous chromosomes. In the wild type, they are impeded from accessing the sister chromatid, presumably by locally activated ASY1 (Hop1), consistent with earlier reports from yeasts (Niu et al, 2005;Carballo et al, 2008;Goldfarb and Lichten, 2010;Latypov et al, 2010). Building on these earlier reports, we infer that this local, DSB-dependent activation is mediated by ATM, because the interhomolog bias of meiotic DNA repair is still intact in atr rad51 mutants.…”

The Mek1 phosphorylation cascade plays a role in meiotic recombination ofSchizosaccharomyces pombe

“…Thus, the recombination reductions may be a consequence of the early role of Mek1 in DSB formation as reported by others. 15,36 Taken together, we conclude that Mek1 plays multiple roles; phosphorylation of Mek1-T15 is important for proper progression of early meiosis including initiation of meiotic S phase, while the kinase and FHA domains of Mek1 are required for proper meiotic recombination.…”

“…This phenotype is noteworthy considering that mek1Δ cells showed no delay in the initiation of meiotic S phase. 15 Next, we analyzed four mek1 mutants, mek1-KD (GP21), -FHAD (TT277), -T15A/KD (TT526) and -T15A/FHAD (TT548); the KD mutant lacks kinase activity and the FHAD mutant lacks the FHA domain function. Notably, the mek1-KD strain (GP21) was not delayed in meiosis (Sup.…”

“…22 Mek1 plays a pivotal role in the meiotic recombination checkpoint 23,24 and appears to enhance homolog interactions to assure WT levels of crossing over. 15 However, knowledge about the position and role of S. pombe Mek1 in the signal transduction cascades during meiosis is limited.…”

“…The FHA domain of Cds1 interacts with the heterodimeric endonuclease Mus81-Eme1, a Holliday junction resolvase, 15 which is required for meiotic crossovers in S. pombe. [25][26][27] Rdh54 (Tid1), a member of the Swi2/Snf2 family of chromatin remodeling proteins, stimulates Dmc1-dependent meiotic recombination and promotes dissociation of Dmc1 from nonrecombinogenic sites on meiotic chromatin.…”