Involvement of Photosystem II in the ammonia metabolism of a heterotrophic cyanobacterium

Herbicides that bind specifically to Photosystem II greatly increased ammonia liberation by a heterotrophic cyanobacterium incubated with methionine sulfoximine anaerobically in light. Comparison of cells incubated under argon and nitrogen indicated that about one-half of the liberated ammonia came from endogenous sources, as well as from dinitrogen fixation. Chromatography of cell extracts revealed a light·induced, general breakdown of cellular proteins in the presence of methionine sulfoximide. Cultures grown on ammonia, and hence free of heterocysts and nitrogenase, liberated ammonia in the dark in nitrogen-free media with methionine sulfoximine and this liberation was inhibited separately by herbicides or light. A combination of light and herbicide, however, also enhanced ammonia liberation by these cells. Herbicidal Photosystem II inhibitors strongly inhibited light·induced assimilation of the ammonia analog, [14 CJmethylamine, by cyanobacteria. These results implicate Photosystem II directly in the ammonia metabolism of this cyanobacterium and suggest that herbicide-binding protein(s) of this system may regulate nitrogen assimilation coordinately with electron transport.