A Nitrate-Inducible Plasma Membrane Protein of a Marine Alga, Heterosigma akashiwo

The rate of nitrate uptake by Heterosigma akashiwo cells that had been cultured in medium with nitrate or ammonium ions as the source of nitrogen was measured using15NO3− The ratio of 15N/14N increased dramatically in nitrate-grown cells. In ammonium-grown cells, the ratio of 15N/14N did not increase for 3 h but then it began to increase. Even when nitrate reductase activity was inhibited by tungstate, nitrate-grown cells could take up nitrate. Plasma membranes from nitrate-grown and ammonium-grown cells were purified by the silica-microbead method, and polypeptides associated with the membranes were analyzed by SDS-PAGE and immunostaining. A major polypeptide with a molecular mass of 26 kDa appeared 3 h after the transfer of ammonium-grown cells to nitrate-containing medium, and it disappeared 2 d after the transfer of nitrate-grown cells to ammonium-containing medium. The 26 kDa polypeptide also appeared when cell growth shifted from the logarithmic phase to the stationary phase and the ammonium content of the medium decreased, even when the cells were cultured in ammonium-containing medium.