Evidence for a H+ nitrate symporter in Aspergillus nidulans.
Nitrate transport in Aspergillus nidulans was dependent upon a consistent proton motive force (delta p) across the cell membrane which was maintained in a range of 105 (+/- 6.7) to 131 (+/- 3.4) mV over an external pH span of 5.5 to 7.5. The membrane potential (delta psi) measured by uptake of [3H]-tetra-phenylphosphonium bromide and the transmembrane pH difference (delta pH) measured by the distribution of 3H2O and [14C]- salicylic acid were used to compute the delta p present during transport of nitrate. Energy dependent accumulation of nitrate was measured in actively assimilating and tungstate inhibited cells. A delta G for nitrate of 14 kJ mol-1 was computed from the results. Cells induced for nitrate transport maintained internal nitrate levels of 6 to 8 mM based on an internal volume of 2.6 microliters/mg dry wt as determined by a conventional dual label procedure. A fivefold higher level of cellular nitrate was observed in tungstate inhibited cells. Nitrate accumulation was dependent upon a H+ gradient which was dissipated by treatment with 2-butanol, the ionophores valinomycin and gramicidin and the proton conductors carbonyl cyanide m-chlorophenyl hydrazone and N,N'-dicyclo-hexylcarbodiimide. Significant ATP and nitrate efflux occurred in cells treated with the above agents. The results suggest that nitrate is transported by symport with H+ on a carrier which is functionally linked to a H+ ATPase pump.
Department of Biological Sciences, Ohio University, Athens 45701.
R J
RJ
Downey
C A
CA
Gedeon
D001233:Aspergillus nidulans / Q000187:drug effects / Q000378:metabolism*; D001693:Biological Transport, Active; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D007476:Ionophores / Q000494:pharmacology; D008564:Membrane Potentials; D009566:Nitrates / Q000378:metabolism*; D017494:Proton Pumps / Q000378:metabolism*; D017973:Tungsten Compounds / Q000494:pharmacology
D007476:Ionophores; D009566:Nitrates; D017494:Proton Pumps; D017973:Tungsten Compounds; C045951:tungstate