Yeast mutants containing elevated methionine

Abstract

Yeasts are deficient in methionine which restricts the nutritive value of the protein. The objective of this research was to produce a methionine-rich mutant. Mutants of Candida tropical is ATCC 1369, Candida tropica!is ATCC 9968, Candida util is ATCC 9950, and Saccharomyces cerevisiae ATCC 862 with increased methionine, the limiting amino acid in yeast protein, content were isolated. None differed from the parent strain as to carbon assimilation or vitamin requirements. The mutants were obtained by treatment with ultraviolet (UV) irradiation, or nitrous acid (NA) or selection for ethionine resistance. The UV-and NA-derived mutants grew on a methionine-free medium and exhibited an increased growth response to sulfate added to a sulfate-free medium. The mutant exhibiting the largest increase in methionine content was 1369/0, an NA mutant, in which methionine was increased 54.6% as a percentage of the protein above that of the parent strain. Of the 17 amino acids recovered, the increase was 9.4%. The increase in the recoverable amino acids was 2%, of which methionine constituted 15%. The increase of the mutant 1369/0 over the parent in relative nutritional value (RNV) was 4.35%, which is of marginal significance. Sonication of the yeast cells increased the RNV an average of 8% above that of the intact cell RNV.