Meiosis is the process of forming haploid cells (spores or gametes) from diploid cells and occurs in all sexually-reproducing organisms. It is accomplished by two successive cell divisions following a single round of DNA replication. Prior to the first meiotic division, recombination usually occurs between homologous chromosomes. This recombination shuffles alleles between maternal and paternal homologs, which serves to maintain genetic diversity in a population. Crossover recombination also forms connections between homologs, which is required in most organisms for the proper segregation of homologous chromosomes at the first meiotic division 
Meiotic recombination is initiated by the formation of double-strand DNA breaks (DSBs), which can be repaired using either a sister chromatid or homologous chromosome as a template 
, only the latter of which gives rise to genetically observable recombination events. DSBs are not uniformly distributed across the chromosomes of most organisms, but occur preferentially at a limited number of sites known as hotspots. Recombination hotspots have been intensively studied in recent years, and the factors determining their distribution in the genome are now emerging. White et al. showed that a hotspot in the promoter of HIS4
requires the transcription factors (TFs) Bas1, Bas2, and Rap1 for hotspot activity 
. The requirement for specific transcription factors also implied the involvement of specific sequence motifs that are recognized by these factors. Later it was shown that Bas1 was involved in the formation of DSBs when bound to its target sequence, TGACTC, at some sites in the genome but not others 
. In the distantly related fission yeast S. pombe
, systematic mutagenesis revealed that a simple sequence, ATGACGT, was necessary for high levels of recombination at the ade6-M26
, which also requires the Atf1-Pcr1 transcription factor for activity 
, indicating that the involvement of TFs may be a widely conserved feature of meiotic recombination hotspots.
The phenomenon of sequence-dependent hotspots of recombination has attracted increased interest recently with the discovery that a 13 bp degenerate motif is responsible for up to 40% of human hotspots 
. That motif, CCNCCNTNNCCNC, is bound by the PRDM9 zinc finger protein that trimethylates lysine 4 of histone H3 (H3K4) 
. H3K4 trimethylation is also required for high-level DSB formation at the majority of hotspots in S. cerevisiae
, though no similar observation has been reported for S. pombe
. In humans, PRDM9 affects recombination not only at sites containing its target sequence, but also those lacking an obvious binding site 
, suggesting that PRDM9 affects recombination both directly, by binding at hotspots, and indirectly, by an as yet unknown mechanism, that is unlikely to include direct binding 
. Therefore, it is possible that the majority of human hotspots are determined by factors other than, or in addition to, PRDM9. These other determinants may include other sequence motifs.
Global analyses of meiotic DSB distributions in both the fission and budding yeasts revealed that the majority of DSBs occur in intergenic regions 
. Since these regions contain promoters, where transcription factors bind to regulate the expression of neighboring genes, this observation is consistent with the hypothesis that many hotspots require specific nucleotide sequences. This model is also consistent with a recent study showing that over 50% of DSBs are located within 500 bp of confirmed TF binding sites 
. Further, at least 30% of DSBs in that study were centered on a TF binding site, suggesting these factors play an integral role in directing Spo11, the protein that forms meiotic DSBs, to those sites. DSBs centered on TF binding sites were categorized as class 1, 2, or 3 depending on whether they showed strong, weak, or no occlusion of DSBs, respectively, around the TF binding site itself (Table S3 in 
). Though few of the TF binding sites analyzed were completely predictive of DSBs, they are certainly among one of several factors, including local and regional chromatin structure, that determine the location of hotspots. Whether they play a causative role in the majority of hotspots has yet to be determined.
Previously, we showed that a large number of different sequences are capable of generating recombination hotspots in S. pombe
. For example, ~0.6% of random 15 bp sequences produced hotspots in the ade6
gene. Assuming that the entire 15 bp sequence is not required for activity, we concluded that approximately 10 seven-bp motifs, or a larger number of eight- or nine-bp motifs, could account for this high frequency. Among the ~500 sequences that produced hotspots, we identified five families of hotspots ≤10 bp in length that occurred multiple times, including the previously identified CRE
family of hotspots 
. Each of these sequences produced a hotspot when reconstructed by minimal base changes to the wild-type ade6
sequence. We also identified transcription factors required for activity of hotspots representing two of these families 
(not including the CRE
family, which is already known to require the Atf1-Pcr1 transcription factor). Based on these results, we proposed that simple sequence motifs could account for the majority, or possibly all, hotspots in S. pombe
and perhaps other organisms. This model was expounded upon by Wahls and Davidson 
, who also noted that our hypothesis could help to resolve the so-called hotspot paradox 
and account for the evolutionarily rapid redistribution of hotspots that has occurred, for example, between chimpanzees and humans 
The question addressed in this study is whether hotspot motifs are organism-specific or conserved across species. Here we show by direct test that four of five hotspots identified in the fission yeast are also active in the budding yeast. Given that these two yeasts are considered as evolutionarily divergent from each other as either is to humans 
, our results suggest that there may be a universal catalog of sequence motifs capable of producing hotspots in most organisms.