MECHANISM OF SODIUM/NITRATE SYMPORT IN ANACYSTIS-NIDULANS R2

A kinetic study of sodium-dependent nitrate transport in the cyanobacterium Anacystis nidulans R2 has been performed by following intracellular accumulation of nitrate in intact cells of the mutant strain FM6, lacking nitrate reductase activity and unable, therefore, to reduce the transported nitrate. Initial transport rates were determined at different external fixed sodium and varying nitrate concentrations and, conversely, at different fixed nitrate and varying sodium concentrations. The resulting kinetic pattern has the best fit to a reaction mechanism model in which sodium nitrate is the substrate for the transporter and sodium behaves additionally as a non-essential activator of the system. Half-saturation constants for the substrate sodium nitrate (1.6 +/- 0.2 mu M) and the activator sodium (0.36 +/- 0.04 mM) have been calculated. The operation of such a sodium/nitrate symport system, driven by the energy of the Delta<(mu)over tilde>(Na+) across the plasma membrane, provides the basis for energy coupling between uphill nitrate and downhill sodium transport into Anacystis cells.