Abstract
1. 1. Excretion of aromatic amino acids was studied in Salmonella typhimurium single aromatic auxotrophs and in analog-resistant mutants that were selected for excretion. Auxotrophs for a single aromatic amino acid excrete one or both of the other aromatic amino acids. Excretion of phenylalanine and tryptophan is prevented by exogenous tyrosine. Tyrosine excretion is prevented by added phenylalanine. A 5-methyltryptophan-resistant mutant excretes tryptophan and this excretion is prevented by tyrosine but not phenylalanine. Two β-2-thienylalanine-resistant strains excrete phenylalanine. Tyrosine prevents excretion in only one. 2. 2. Regulation of phenylalanine-, tyrosine- and tryptophan-controlled enzymes was studied. Tyrosine-inhibited DAHP synthase (7-phosho-2-keto-3-deoxy- D - arabino -heptonate D -erythrose-4-phosphate-lyase (pyruvate-phosphorylating), EC 4.1.2.15) was more responsive to repression than the phenylalanine-inhibited isoenzyme. Depression of tyrosine-inhibited DAHP synthase and prephanate dehydrogenase (prephenate: NAD oxidoreductase (decarboxylating)) was about 20-fold. Under conditions of 4-fold derepression of prephenate dehydratase (prephenate hydro-lyase (decarboxylating)) the level of phenylalanine-inhibited DAHP synthase, remained constant. Total chorismate mutase activity was derepressed about 8-fold. Anthranilate synthetase, which served as an indicator for the tryptophan operon was derepressed over 60-fold. 3. 3. Possible explanations are given for loss of regulation that results in aromatic amino acid excretion by auxotrophs and by analog-resistant mutants.
Published Version
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