The effect of the mating-type alleles on intragenic recombination in yeast.

alleles a and CY govern the mating reaction in Saccharomyces cereuisiae T y E ~ ~ ~ ~ ~ ~ ~ and LINDEGREN 1943). High-frequency mating is obtained when haploid a or diploid a a cells are mixed with CY or 01 CY cells (ROMAN, PHILLIPS, and SANDS 1955). Matiings of cells of like mating type are rare and have been observed, in fact, only between CY cells. The mating type alleles also govern spomlation. Cells that have at least one a and one CY allele can be induced to sporulate, whereas a a and 01 CY celjls do not sporulate under any of the conditions that have been tried. Intragenic recombination occurs spontaneously in a O( diploids and its frequency can be increased substantially with ultraviolet light (ROMAN and JACOB 1957). Thus there is reason to believe that chromosomal pairing can occur during mitosis in cells of this type (see also WILKIE and LEWIS 1963; HURST and FOGEL 1964). The fact that a a and 01 ,T cells do not sporulate suggests the possibility that such cells are defective in the meiotic process. Mitotic intragenic recombination should not be affected if the lesion does not interfere with chromosomal pairing and exchange. If the lesion prevents or inhibits pairing and exchange, a reduction in intragenic recombination would be expected. The experiments described below were designed to test these alternatives.