Temperature-Sensitive Mutants of Saccharomyces cerevisiae Variable in the Methionine Content of Their Protein

Temperature-sensitive mutants were derived fromSaccharomyces cerevisiae Y5α by ethyl methane sulfonate mutagenesis, in a search for mutants that would produce methionine-rich protein at the nonpermissive temperature. A total of 132 mutant strains were selected which showed adequate growth on minimal medium at 25°C but little or no growth on the same medium supplemented with a high concentration (2 mg/ml) ofl -methionine at 37°C. Several of these mutants were found to increase the proportion of methionine in their protein to much higher levels than that of the wild-type parent after a temperature shift from 25 to 37°C. Two strains, 476 and 438, which were temperature sensitive only in the presence of methionine, produced cellular protein with methionine contents as high as 3.6 and 4.3%, respectively, when incubated in the presence of methionine. The former strain contained 2.5% methionine even when incubated at 37°C in the absence of methionine. Wild strain Y5α, on the other hand, had 1.75% methionine under all conditions tested. Most temperature-sensitive mutants isolated had the same methionine content as the wild strain. It is concluded that the proportion of a specific amino acid, such as methionine, inS. cerevisiae protein can be altered by culturing certain temperature-sensitive mutants at an elevated temperature.