Hot spots of recombination in fission yeast: inactivation of the M26 hot spot by deletion of the ade6 promoter and the novel hotspot ura4-aim.

The M26 mutation in the ade6 gene of Schizosaccharomyces pombe creates a hot spot of meiotic recombination. A single base substitution, the M26 mutation is situated within the open reading frame, near the 5' end. It has previously been shown that the heptanucleotide sequence 5' ATGACGT 3', which includes the M26 mutation, is required for hot spot activity. The 510-bp ade6-delXB deletion encompasses the promoter and the first 23 bp of the open reading frame, ending 112 bp upstream of M26. Deletion of the promoter in cis to M26 abolishes hot spot activity, while deletion in trans to M26 has no effect. Homozygous deletion of the promoter also eliminates M26 hot spot activity, indicating that the heterology created through deletion of the promoter per se is not responsible for the loss of hot spot activity. Thus, DNA sequences other than the heptanucleotide 5' ATGACGT 3', which must be located at the 5' end of the ade6 gene, appear to be required for hot spot activity. While the M26 hotspot stimulates crossovers associated with M26 conversion, it does not affect the crossover frequency in the intervals adjacent to ade6. The flanking marker ura4-aim, a heterology created by insertion of the ura4+ gene upstream of ade6, turned out to be a hot spot itself. It shows disparity of conversion with preferential loss of the insertion. The frequency of conversion at ura4-aim is reduced when the M26 hot spot is active 15 kb away, indicating competition for recombination factors by hot spots in close proximity.