Apparent genetic redundancy facilitates ecological plasticity for nitrate transport

Aspergillus nidulans possesses two high-affinity nitrate transporters, enco ded by the nrtA and the nrtB genes. Mutants expressing either gene grew nor mally 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 re gulatory genetic backgrounds studied. Flux analysis of single gene mutants using (NO3-)-N-13 revealed that K-m values for the NrtA and NrtB transporte rs were similar to 100 and similar to 10 muM, respectively, while V-max val ues, though variable according to age, were similar to 600 and similar to 1 00 nmol/mg dry weight/h, respectively, in young mycelia. This kinetic diffe rentiation may provide the necessary physiological and ecological plasticit y to acquire sufficient nitrate despite highly variable external concentrat ions. Our results suggest that genes involved in nitrate assimilation may b e induced by extracellular sensing of ambient nitrate without obligatory en try into the cell.