Enzyme derepression and feedback inhibition of the first
enzyme are the regulatory mechanisms demonstrated for the
tryptophan pathway in Saccharomyces cerevisiae. The relative contributions of the two mechanisms to the control of the flux through the pathway in vivo were analyzed by (i) measuring feedback inhibition of
anthranilate synthase in vivo, (ii) determining the effect of regulatory mutations on the level of the
tryptophan pool and the flux through the pathway, and (iii) varying the gene dose of individual
enzymes of the pathway at the
tetraploid level. We conclude that the flux through the pathway is adjusted to the rate of
protein synthesis by means of feedback inhibition of the first
enzyme by the end product,
tryptophan. The synthesis of the
tryptophan enzymes could not be repressed below a basal level by
tryptophan supplementation of the media. The
enzymes are present in excess. Increasing or lowering the concentration of individual
enzymes had no noticeable influencing on the overall flux to
tryptophan. The uninhibited capacity of the pathway could be observed both upon relieving feedback inhibition by
tryptophan limitation and in feedback-insensitive mutants. It exceeded the rate of consumption of the
amino acid on minimal medium by
a factor of three.
Tryptophan limitation caused derepression of four of the five
tryptophan enzymes and, as a consequence, led to a further increase in the capacity of the pathway. However, because of the large reserve capacity of the "repressed" pathway,
tryptophan limitation could not be imposed on wild-type cells without resorting to the use of analogs. Our results, therefore, suggest that derepression does not serve as an instrument for the specific regulation of the flux through the
tryptophan pathway.