Nitrite transport is mediated by the nitrite-specific high-affinity NitA transporter and by nitrate transporters NrtA, NrtB in Aspergillus nidulans

Disruption of the Aspergillus nidulans high-affinity nitrate transporter genes (nrtA and nrtB) prevents growth on nitrate but not nitrite. We identified a distinct nitrite transporter (Km = 4.2 ± 1 μM, Vmax = 168 ± 21 nmol mg−1 DW−1 h−1), designated NitA. Disruption of nrtA, nrtB and nitA blocked growth on nitrite, despite low rates of nitrite depletion we ascribe to passive nitrous acid permeation. Growth of the single mutant nitA16 on nitrite was wild-type, suggesting that NrtA and/or NrtB transports nitrite as well as nitrate. Indeed, NrtA and NrtB transport nitrite at higher rates than NitA; Km and Vmax values were 16 ± 4 μM and 808 ± 67 nmol mg−1 DW−1 h−1 (NrtA) and 11 ± 1 μM and 979 ± 17 nmol mg−1 DW−1 h−1 (NrtB). We suggest that NrtA is a nitrate/nitrite transporter, NrtB absorbs nitrite in preference to nitrate and NitA is exclusively a nitrite transporter.