Tryptophan transport in Neurospora crassa. II. Metabolic control.

The rate of tryptophan transport in Neurospora is regulated by the intracellular pool of tryptophan. When cells were shifted from growth in minimal medium to tryptophan-containing medium for 10 min, there was a 50% reduction in the rate of tryptophan transport. Intracellular tryptophan pools derived from indole were equally effective in reducing the rate of transport as externally supplied tryptophan. The regulatory influence of tryptophan on the transport system appears to be a property of all the amino acids transported by the tryptophan transport site or sites. Lysine and glutamic acid are not transported by the tryptophan transport site or sites and are ineffective in the regulation of tryptophan uptake. Continued protein synthesis is required for the maintenance of a functional tryptophan transport system. The half-life of the transport system, estimated by inhibiting protein synthesis with cycloheximide, was about 15 min. Turnover of the system occurred at 30 C but not at 4 C, suggesting that the breakdown of the system is enzymatically mediated. It was inferred that the rate of tryptophan transport in Neurospora is modulated through the maintenance of a delicate balance between the synthesis and breakdown of some component of the transport system.