Contribution of nitrate to H+ permeability of Synechococcus PCC 6311 plasmalemma vesicles

The fluorescent pH-sensitive probe 9-aminoacridine was used to show the contribution of nitrate to proton permeability of purified plasmalemma vesicles from the cyanobacterium Synechococcus PCC 6311. Kinetics of the fluorescence quenching of 9-aminoacridine were performed with a proton gradient of 1.5 ΔpH across the plasma membrane, acidic inside, in the presence of different salts (NaNO3, KNO3, NaCl, KCl) inside or outside the plasmalemma vesicles. It was observed that the proton gradient dissipation was accelerated when the NO3− anion was in the same compartment as H+, whatever the cation. When the salt was external, NaNO3 had a more efficient effect than KNO3 on the H+ gradient dissipation, which could be explained by the H+/Na+ antiport system present in this membrane. Analysis of the data suggested the existence of an H+/NO3− symport.