Genetic improvement and optimization of Saccharomyces cerevisiae for high yield methionine

Yeast is an interesting source of microbial protein for human and animals. One problem in using yeast as protein source is that it is low in methionine content but high in nucleic acid which cause potential risks for health. In order to increase protein quality and reduce the amount of yeast use as methionine and protein source methionine-rich mutant was constructed by UV mutation. Cells of S. cerevisiae N1, a selected strain confered high protein and methionine content in its cell, were treated by UV-irradiation and then ethionine resistant and methionine-rich mutant was selected. Among 3,200 colonies of survival cells, a mutant S. cerevisiae NJ-42, contained the highest amount of methionine in its cell which also higher than its wild type, S. cerevisiae N1. When the mutant NJ-42 was cultivated in 150 ml of minimal medium containing 1.0 percent glucose, 0.05 percent ammonium phosphate and pH 5.0 in 500 ml Erlenmeyer flask and incubated on rotary shaker, 170 rpm, at room temperature for 72 h, cells of the mutant contained 4.412 mg methionine/g dry weight. While at optimal conditions, cells of S. cerevisiae N1 contained only 0.584 mg methionine/g dry weight when cultivated in minimal medium containing 1.0 percent glucose, ammonium sulphate 0.05 percent, pH 6.0, initial cell concentration as optical density at 550 nm was 1.0 and incubated at 28 deg C on rotary shaker, 170 rpm, for 48 h. The mutant S. cerevisiae NJ-42 was subjected to double mutation by UV-irradiation and 200 mutants was randomly selected based on their growth on minimal medium containing ethionine 400 micro g/ml. Among these mutants 11 mutants confered higher methionine content than the mutant NJ-42 and a mutant S. cerevisiae MR-195 contained the highest amount of methionine. Under the optimal conditions for maximum accumulation of methionine of the mutant MR-195, minimal medium containing 1.0 percent glucose, 0.02 percent ammonium phosphate, pH 6.0, initial cell concentration as optical density at 550 nm was 1.5 and incubated at 28 deg C on rotary shaker, 170 rpm at 24 h, the methionine content of the MR-195 was 19.438 mg methionine/g dry weight which was 33.28 times higher than S. cerevisiae N1, under its optimal conditions. When the mutant MR-195 was cultivated in 5 l jar-fermenter with 3 l working volume using the same medium with 500 rpm agitation speed and 1.0 vvm aeration rate, the highest methionine content of 11.76 mg methionine/g dry weight was obtained after 12 h.