EVIDENCE FOR A H-NIDULANS( NITRATE SYMPORTER IN ASPERGILLUS)

Nitrate transport in Aspergillus nidulans was dependent upon a consist ent proton motive force (Delta p) across the cell membrane which was m aintained in a range of 105 (+/- 6.7) to 131 (+/- 3.4) mV over an exte rnal pH span of 5.5 to 7.5. The membrane potential (Delta Psi) measure d by uptake of [H-3]-tetra-phenylphosphonium bromide and the transmemb rane pH difference (Delta pH) measured by the distribution of (H2O)-H- 3 and [C-14]-salicylic acid were used to compute the Delta p present d uring transport of nitrate. Energy dependent accumulation of nitrate w as measured in actively assimilating and tungstate inhibited cells. A Delta G for nitrate of 14 kJ mol(-1) was computed from the results. Ce lls induced for nitrate transport maintained internal nitrate levels o f 6 to 8 mM based on an internal volume of 2.6 mu l/mg dry wt as deter mined 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 a nd the proton conductors carbonyl cyanide m-chlorophenyl hydrazone and N,N'-dicyclo-hexylcarbodiimide. Significant ATP and nitrate efflux oc curred in cells treated with the above agents. The results suggest tha t nitrate is transported by symport with H+ on a carrier which is func tionally linked to a H+ ATPase pump.