| Abstract: | Kinetic parameters for three systems of active histidine uptake by germinated conidia of Neurospora crassa have been measured. Each system appears to follow typical Michaelis-Menten kinetics when studied separately from the other systems. Under the conditions studied, the general amino acid transport system was found to account for the major portion of histidine uptake from low concentrations. Three types of transport mutants with altered growth inhibition patterns were selected in a histidine auxotroph. Growth of one mutant, type bas(a), could be inhibited by the addition of methionine to a histidine-supplemented medium, and another type, neu(a), could be inhibited by the addition of arginine. These mutants were shown to be lacking active histidine uptake by the basic amino acid and neutral amino acid transport systems, respectively. Another type of double mutant (his-3, neu(r)) could be inhibited only by the addition of very high concentrations of methionine in the presence of arginine and histidine, and the mutation appeared to have altered the specificity of the neutral amino acid permease. |
