Physiological and regulatory properties of the general amino acid transport system of Neurospora crassa

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Abstract
The fundamental properties of the general amino acid transport system of N. crassa were investigated in the conidial stage of the life cycle. The transport activity was under genetic control, and an isogenic set of mutants deficient for the neutral, basic or general amino acid transport systems and combinations thereof was constructed and used for analyzing the properties specific to the general permease. Amino acid transport by this system was a carrier-mediated active process with broad specificity for the neutral and basic amino acids. Kinetic analysis revealed that a common binding site functioned to transport both neutral and basic amino acids and that the permease had a high affinity for its substrates. The kinetic parameters Km, Vmax and Ki [inhibition constant] were defined for several substrates. Two modes of regulation were detected. substrate inhibition and ammonium repression. Activity of the general system was enhanced by the removal of NH4+ from the incubation medium with a concomitant decline in either neutral or basic permease activity, suggesting that a common component exists between the neutral and the general systems and between the basic and the general systems.

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