Wolfgang Duntze

Isolation and characterization of Saccharomyces cerevisiae mutants supersensitive to G1 arrest by the mating hormone a-factor

SummaryNine independent mutants which are supersensitive (ssl−) to G1 arrest by the mating hormone a-factor were isolated by screening mutagenized Saccharomyces cerevisiae MATα cells on solid medium for increased growth inhibition with a-factor. These mutants carried lesions in two complementation groups, ssl1 and ssl2. Mutations at the ssl1 locus were mating type specific: MATα ssl1− cells were supersensitive to α-factor but MATα ssl1− were not supersensitive to α-factor. In contrast, mutations at the ssl2. locus conferred supersensitivity to the mating hormone of the opposite mating type on both MATα, and MATa cells. The α-cell specific capacity to inactivate externally added a-factor was shown to be lacking in MATα ssl1− mutants whereas MATα ssl2. cells were able to inactivate a-factor. Complementation analysis showed that ssl2 and sst2, a mutation originally isolated as conferring supersensitivity to α-factor to MATa cells, are lesions in the same gene. The ssl1 gene was mapped 30.5 centi-Morgans distal to ilv5 on chromosome XII.

Pheromones in Yeasts

Many species of yeast possess life cycles which can be considered as primitive sexual cycles with alternating haploid and diploid growth phases during which the cells multiply vegetatively. Transition from the diplophase to the haplophase is achieved by a nuclear reduction division (meiosis). The four meiotic products are generally incorporated into spores which under appropriate conditions may germinate to form vegetatively growing haploid cells. The diplophase is restored by conjugation of two haploid cells which in heterothallic (self-incompatible) species belong to differentiated mutually exclusive mating types exhibiting distinct physiological characteristics. Two cells of opposite mating type fuse to form a zygote in which the diploid state is reestablished by fusion of the two haploid nuclei.