Role of DNA Sequences in Genetic Recombination in the Iso-1-Cytochrome c Gene of Yeast. I. Discrepancies between Physical Distances and Genetic Distances Determined by Five Mapping Procedures

Recombination rates have been examined in two-point crosses of various defined cyc1 mutants using five mapping methods. Nucleotide sequences of mutant codons were identified in previous studies from alterations in functional iso-1-cytochromes c produced by intragenic revertants. Heteroallelic diploids were analyzed for rates of mitotic recombination that occurred spontaneously and that were induced with x-rays, ultraviolet light and the near-ultraviolet light emitted by sunlamps, as well as rates of meiotic recombination that occur after sporulation. Frequencies of both mitotic and meiotic recombination do not necessarily correspond with physical distances separating altered nucleotides. The most extreme discrepancy involved two adjacent intervals of thirteen basepairs which differed approximately thirty-fold in their spontaneous and X-ray-induced recombination rates. Marked disproportions between genetic and physical distances appear to be due to the interaction of the two nucleotide sequences in the heteroallelic combination and not to the sequences of the mutant codons alone. Recombination values that were obtained by all five methods could not be used to establish to correct order of mutant sitesmrelationships of the recombination rates for the various pairwise crosses are different after mitosis from those after meiosis, suggesting that these two recombinational processes are to some extent different in their dependence on particular nucleotide configurations. On the other hand, the relationships of the rates induced by UV-, sunlamp- and X-irradiation were identical or very similar. In addition to the intrinsic properties of the alleles affecting frequencies of mitotic and meiotic recombination rates, two- to threefold variations in recombination rates could be attributed to genetic backgrounds.