A derepressible active transport system for glucose in Neurospora crassa.

Two glucose uptake systems have been detected in ungerminated conidia of wild type Neurospora crassa, strain 74A; the high &ty system has been characterized. Uptake and substrate competition experiments showed that the high &inity system was relatively speciiic for D-glucose but also trsnsported the nonmetabolizable analogues 6-deoxy-D&ucase, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose. The system exhibited a K, of 0.07 mma nd a V,,,,, of 4 mqoles per mg, dry weight, per min for 3-O-methyl-D-glucose uptake. The nonmetabolizable analogues were accumulated against concentration gradients; chromatographic analysis showed that the material accumulated in each case was the free sugar. Uptake by the high affinity system was inhibited by azide and dinitrophenol. These results were consistent with an active transport mechanism for uptake. Two alternative mechanisms, group translocation and coupled transpor$ were considered unlikely because no transitory substrate derivative could be detected during uptake and because inlh of 3-omethyl-D-glucose was not coupled to ef&ux of any organic metabolite. The high afihrity system was repressed when the conidia were germinated on glucose medium but was restored when the glucose was either depleted or removed from the medium. This restoration was inhibited by cycloheximide. Two additional kinetic features were of importance in interpreting 3-O-methyl-D-glucose transport. First, previously accumulated substrate stimulated the subsequent influx of 3-O-methyl-D-glucose. The stimulation was proportional to the internal substrate concentration and was inhibited by azide. Second, eflhx appeared to be a carrier-mediated process, but, in contrast to the high athnity uptake system, it was not inhibited by azide or by sulfhydryl reagents.