Gene conversion in nonsense suppressors of Schizosaccharomyces pombe : II. Specific marker effects.

Gene conversion and postmeiotic segregation patterns have been analysed at 14 mutant sites of sup3, sup8 and sup9 including 5 alleles with a strong marker effect on recombination frequencies in two-factor crosses. The total frequency of gene conversion and postmeiotic segregation tetrads is fairly constant within each gene, but may vary from one gene to another. About 97% of the conversion events are coconversions spanning the whole sup gene. Postmeiotic segregations are usually quite rare. None of the marker-effect alleles has an increased rate of hybrid DNA formation at the allele considered, as judged from the frequency of gene conversion and postmeiotic segregation in one-factor crosses. At least two of them, sup3-e and sup9-e, are associated with a high frequency of postmeiotic segregation indicating a poor repair of the corresponding base-pair mismatches. This is also observed in a two-factor cross and can account for the marker effect on recombination frequencies. The properties of a third marker effect allele, sup3-e,r10, are best explained by a higher probability of single site conversions as opposed to coconversions in two-factor crosses involving the mutant site r10.