The ftr cistron of Coprinus cinereus is the structural gene for a multifunctional transport carrier molecule

Mutants of the basidiomycete Coprinus cinereus which were selected for their resistance to growth inhibitions caused by hexose analogues are all alleles of the ftr cistron. They are shown to have approximately normal levels of activity of enzymes involved in intracellular sugar metabolism and to accumulate normal levels of sugar phosphates. However, the mutants show greatly depressed rates of sugar uptake. Uptake rates from 0.01 mM solutions of 2-deoxy-D-glucose were only 1 to 4% of the wild type rate, and from 15 mM solutions the mutant rates were between 16 and 40% of normal. Kinetic analysis showed that the mutant Vmax values were reduced to a few per-cent of normal while Km values were relatively little changed and in some cases the mutants had an increased affinity for the substrate. Reverse mutations restored the Vmax value and the Km to about the wild type level. Previous data had shown that position of mutants within the allele map depended on selection conditions in a way that implied some interaction between the ftr gene product and the substrate. Since the mutants are defective in transport from both high and low sugar concentrations, and since they exhibit coordinated alterations in Km and Vmax, it is concluded that the ftr cistron is the structural gene for a product involved in sugar translocation (both as carrier and energisation link) in both high and low affinity glucose transport systems.