No abstract

SummaryNitrite-specific plasma membrane transporters have been described in bacteria, algae and fungi, but there is no evidence of a nitrite-specific plasma membrane transporter in higher plants. We have used 13 NO À 2 to characterize nitrite influx into roots of Arabidopsis thaliana. Hydroponically grown Arabidopsis mutants, defective in high-affinity nitrate transport, were used to distinguish between nitrate and nitrite uptake by means of the short-lived tracers 13 NO À 2 and 13 NO À 3 . This approach allowed us to characterize a nitrite-specific transporter. The Atnar2.1-2 mutant, lacking a functional high-affinity nitrate transport system, is capable of nitrite influx that is constitutive and thermodynamically active. The corresponding fluxes conform to a rectangular hyperbola, exhibiting saturation at concentrations above 200 lM (K m = 185 lM and V max = 1.89 lmol g À1 FW h À1

Summary• Here, influx and efflux of , and net fluxes of and , were measured in Aspergillus nidulans mutants niaD171 and niiA5 , devoid of nitrate reductase (NR) and nitrite reductase (NiR) activities, respectively.• Transcript and protein abundances of NrtA, the A. nidulans principal high-affinity transporter, were determined using semiquantitative reverse transcriptionpolymerase chain reaction and western blots, respectively.• influx in niaD171 was negligible relative to wild-type values, whereas efflux to influx ratios increased nine-fold. Nevertheless, NrtA mRNA and NrtA protein were expressed at levels more than two-fold and three-fold higher, respectively, in niaD171 than in the wild-type strain.• This is the first demonstration of diminished high-affinity influx associated with elevated transporter levels, providing evidence that, in addition to transcriptional regulation, control of NrtA expression operates at the post-translational level. This mechanism allows for rapid control of transport at the protein level, reduces the extent of futile cycling of that would otherwise represent a significant energy drain when influx exceeds the capacity for assimilation or storage, and may be responsible for the rapid switching between the on and off state that is associated with simultaneous provision of to mycelia absorbing .