PREFERENTIAL STRAND TRANSFER AND HYBRID DNA FORMATION AT THE RECOMBINATION HOTSPOT ADE6-M26 OF SCHIZOSACCHAROMYCES-POMBE

The ade6-M26 mutation of Schizosaccharomyces pombe stimulates intragen ic and intergenic meiotic recombination. M26 is a single base pair cha nge creating a specific heptanucleotide sequence that is crucial for r ecombination hotspot activity. This sequence is recognized by proteins that may facilitate rate-limiting steps of recombination at the ade6 locus. To start the elucidation of the intermediate DNA structures for med during M26 recombination, we have analyzed the aberrant segregatio n patterns of two G to C transversion mutations flanking the heptanucl eotide sequence in crosses homozygous for M26. At both sites the level of post-meiotic segregation is typical for G to C transversion mutati ons in S.pombe in general. Quantitative treatment of the data provides strong evidence for heteroduplex DNA being the major recombination in termediate at the M26 site. We can now exclude a double-strand gap rep air mechanism to account for gene conversion across the recombination hotspot, Furthermore, the vast majority (>95%) of the heteroduplexes c overing either of the G to C transversion sites are produced by transf er of the transcribed DNA strand. These results are consistent with ad e6-M26 creating an initiation site for gene conversion by the introduc tion of a single-strand or a double-strand break in its vicinity, foll owed by transfer of the transcribed DNA strands for heteroduplex DNA f ormation.