M26 RECOMBINATIONAL HOTSPOT AND PHYSICAL CONVERSION TRACT ANALYSIS INTHE ADE6 GENE OF SCHIZOSACCHAROMYCES-POMBE

At the ade6 locus of Schizosaccharomyces pombe flanking markers have b een introduced as well as five silent restriction site polymorphisms: four in the 5' upstream region and one in the middle of the gene. The mutations ade6-706, ade6-M26 (both at the 5' end) and ade6-51 (middle of the gene) were used as partners for crosses with the 3' mutation ad e6-469. From these three types of crosses, wild-type recombinants were selected and analyzed genetically to assess association with crossing -over and physically to determine conversion tract lengths. The introd uced restriction site polymorphisms (five vs. only one) neither influe nced the pattern of recombinant types nor the distribution of conversi on tracts. The hotspot mutation M26 enhances crossing-over and convers ion to the same proportion. M26 not only stimulates conversion at the 5' end, but does this also (to a lower extent) at the 3' end of ade6 a t a distance of more than 1 kb. The majority of meiotic conversion tra cts are continuous and postmeiotic segregation of polymorphic sites is rare. Conversion tracts are slightly shorter with M26 in comparison w ith its control 706. The mean minimal length of tracts varies from 670 bp (M26) to 890 bp (706) to 1290 bp (51). It is concluded that M26 ac ts as an initiation site of recombination or enhances initiation of re combination. M26 does not act by termination of conversion. A region o f recombination initiation exists at the 5' end of the ade6 gene also in the absence of the ade6-M26 hotspot mutation.