Tryptophan transport in Neurospora crassa. I. Specificity and kinetics

Wiley, W. R. (Battelle Memorial Institute, Richland, Wash.), and W. H. Matchett. Tryptophan transport in Neurospora crassa. I. Specificity and kinetics. J. Bacteriol. 92:1698-1705. 1966.-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 Q(10) for the process is approximately 2 between 20 and 30 C. The apparent K(m) for uptake is 5 x 10(-5)m. Leucine and phenylalanine competitively inhibit the rate of tryptophan transport; the K(i) 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 alpha amino group, next to a carboxyl, represent three attachment points for the uptake site.