SHORT COMMUNICATION Isolation and Genetical Classification of Aromatic Amino Acid

Mutants of Schizosaccharomyces pombe with single and multiple requirements for aromatic amino acids were isolated and allocated to 15 loci. Difficulties in cultivation of many of these auxotrophs were largely overcome by using a minimal medium with L-glutamic acid as sole nitrogen source. The locus-typical growth factor requirements of mutants allowed a preliminary allocation of genes to reaction sequences in aromatic amino acid biosynthesis. Several explanations for the occurrence of mutants with locus-atypical growth factor requirements are discussed. I N T R O D U C T l O N We have attempted to isolate mutants of Schizosaccharomyces pombe auxotrophic for aromatic amino acids and aromatic vitamins. Mutants blocked in enzymes of the trypto-phan-synthesizing branch of the pathway (Schweingruber & Dietrich, 1973) and others defective in isoenzymes controlling the first step of aromatic amino acid biosynthesis (Schweingruber & Wyssling, 1974) have already been isolated and characterized biochemi-cally, but no mutants with multiple requirements for aromatic amino acids, as have been described in Neurospora crassa (Giles et al., 19671, Saccharomyces cerevisiae (De Leeuw, 1968) and Aspergillus nidulans (Roberts, 1969), have been isolated. Here we report on the isolation and genetical classification of such 5. pombe mutants as well as of others with single requirements for phenylalanine, tyrosine, tryptophan or p-aminobenzoic acid. METHODS Strains. The following strains of Schizosaccharomyces pombe stem from the silica gel stock collectio