Aspergillus nidulans possesses two high‐affinity nitrate transporters, encoded by the nrtA and the nrtB genes. Mutants expressing either gene grew normally on 1–10 mM nitrate as sole nitrogen source, whereas the double mutant failed to grow on nitrate concentrations up to 200 mM. These genes appear to be regulated coordinately in all growth conditions, growth stages and regulatory genetic backgrounds studied. Flux analysis of single gene mutants using 13NO3− revealed that Km values for the NrtA and NrtB transporters were ∼100 and ∼10 μM, respectively, while Vmax values, though variable according to age, were ∼600 and ∼100 nmol/mg dry weight/h, respectively, in young mycelia. This kinetic differentiation may provide the necessary physiological and ecological plasticity to acquire sufficient nitrate despite highly variable external concentrations. Our results suggest that genes involved in nitrate assimilation may be induced by extracellular sensing of ambient nitrate without obligatory entry into the cell.

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