Isolation and characterization of temperature-sensitive mutations in the RAS2 and CYR1 genes of Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae contains two ras homologues, RAS1 and RAS2, whose products have been shown to modulate the activity of adenylate cyclase encoded by the CYR1 gene. To isolate temperature-sensitive mutations in the RAS2 gene, we constructed a plasmid carrying a RAS2 gene whose expression is under the control of the galactose-inducible GAL1 promoter. A ras1 strain transformed with this plasmid was subjected to ethyl methanesulfonate mutagenesis and nystatin enrichment. Screening of approximately 13,000 mutagenized colonies for galactose-dependent growth at a high temperature (37 degrees) yielded six temperature-sensitive ras2 (ras2ts) mutations and one temperature-sensitive cyr1 (cyr1ts) mutation that can be suppressed by overexpression or increased dosage of RAS2. Some ras2ts mutations were shown to be suppressed by an extra copy of CYR1. Therefore increased dosage of either RAS2 or CYR1 can suppress the temperature sensitivity caused by a mutation in the other. ras1 ras2ts and ras1 cyr1ts mutants arrested in the G1 phase of the cell cycle at the restrictive temperature, and showed pleiotropic phenotypes to varying degrees even at a temperature permissive for growth (25 degrees), including slow growth, sporulation on rich media, increased accumulation of glycogen, impaired growth on nonfermentable carbon sources, heat-shock resistance, impaired growth on low concentrations of glucose, and lithium sensitivity. Of these, impaired growth on low concentrations of glucose and sensitivity to lithium are new phenotypes, which have not been reported for mutants defective in the cAMP pathway.