The transport of tryptophan in Neurospora crassa is mediated by a distinct stereospecific system which is chemically specific for a family of neutral amino acids. The process shows typical saturation kinetics and a sharp decrease in the rate of tryptophan uptake at low temperatures. The Qlo for the process is approximately 2 between 20 and 30 C. The apparent Km for uptake is 5 X 10-5 M. Leucine and phenylalanine competitively inhibit the rate of tryptophan transport; the Ki values are 1.1 X 10-4 M and 4.0 x 10-5 M, respectively. These data are interpreted as evidence that these amino acids are transported by the same transport site(s). Inhibition studies with amino acids and other compounds structurally related to leucine and phenylalanine suggest that an uncharged side chain and an a amino group, next to a carboxyl, represent three attachment points for the uptake site.

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The accumulation of imidazoleglycerol phosphate during growth of Neurospora crassa in the presence of 3-amino-1 , 2,4-triazole was found to cause derepression of tryptophan synthetase and to inhibit the induction of kynureninase. Accumulation of indoleglycerol phosphate in response to growth in the presence of indole acrylic acid or anthranilic acid was also accompanied by derepressed synthesis of tryptophan synthetase. Enzyme synthesis in mutants (his-7 and trp-4) unable to form these intermediates was not altered under similar conditions. The rate of formation of tryptophan synthetase and kynureninase was found to differ in the presence of tryptophan and indole.

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Induction of kynureninase in a homogeneous population of mycelia in balanced growth was studied. Induced synthesis of enzyme was apparent 10 min after addition of inducers (tryptophan, kynurenine, and N -formylkynurenine). Experiments with cycloheximide-inhibited cells indicated that initiation of message transcription occurred about 4 min after addition of inducers. Lifetime of the message in the absence of inducer was of the order of 5 min. Experiments with mutants showed that N -formylkynurenine was the probable inducer of physiological importance.