Selection of lys2 mutants of the yeast Saccharomyces cerevisiae by the utilization of alpha-aminoadipate

Normal strains of Saccharomyces cerevisiae do not use alpha-aminoadipate as a principal nitrogen source. However, alpha-aminoadipate is utilized as a nitrogen source by lys2 and lys5 strains having complete or partial deficiencies of alpha-aminoadipate reductase and, to a limited extent, by heterozygous lys2/+ strains. Lys2 mutants were conveniently selected on media containing alpha-aminoadipate as a nitrogen source, lysine, and other supplements to furnish other possible auxotrophic requirements. The lys2 mutations were obtained in a variety of laboratory strains containing other markers, including other lysine mutations. In addition to the predominant class of lys2 mutants, low frequencies of lys5 mutants and mutants not having any obvious lysine requirement were recovered on alpha-aminoadipate medium. The mutants not requiring lysine appeared to have mutations at the lys2 locus that caused partial deficiencies of alpha-aminoadipate reductase. Such partial deficiencies are believed to be sufficiently permissive to allow lysine biosynthesis, but sufficiently restrictive to allow for the utilization of alpha-aminoadipate. Although it is unknown why partial or complete deficiencies of alpha-aminoadipate reductase cause utilization of alpha-aminoadipate as a principal nitrogen source, the use of alpha-aminoadipate medium has considerable utility as a selective medium for lys2 and lys5 mutants.