PHENYLALANINE-AUXOTROPHIC AND TYROSINE-AUXOTROPHIC MUTANTS OF SACCHAROMYCES-CEREVISIAE IMPAIRED IN TRANSAMINATION

This paper reports the first isolation of Saccharomyces cerevisiae mut ants lacking aromatic aminotransferase I activity (aro8), and of aro8 aro9 double mutants which are auxotrophic for both phenylalanine and t yrosine, because the second mutation, aro9, affects aromatic aminotran sferase II. Neither of the single mutants displays any nutritional req uirement on minimal ammonia medium. In vitro, aromatic aminotransferas e I is active not only with the aromatic amino acids, but also with me thionine, alpha-aminoadipate, and leucine when phenylpyruvate is the a mino acceptor, and in the reverse reactions with their ore-acid analog ues and phenylalanine as the amino donor. Its contribution amounts to half of the glutamate:2-oxoadipate activity detected in cell-free extr acts and the enzyme might be identical to one of the two known alpha-a minoadipate aminotransferases. Aromatic aminotransferase I has propert ies of a general aminotransferase which, like several aminotransferase s of Escherichia coli, may be able to play a role in several otherwise unrelated metabolic pathways. Aromatic aminotransferase II also has a broader substrate specificity than initially described. In particular , it is responsible for all the measured kynurenine aminotransferase a ctivity. Mutants lacking this activity grow very slowly on kynurenine medium.