In vivo short‐term inhibition of nitrogenase by nitrate in Rhodopseudomonas capsulata E1F1

Rhodopseudomonas capsulata E1F1, a purple non‐sulfur bacterium capable of photoassimilating nitrate, could fix N 2 in the presence of nitrate until nitrite traces originated by nitrate reduction were excreted to the medium. Nitrite excretion resulted in the immediate cessation of dinitrogen fixation that was resumed once nitrite was assimilated. Although nitrite addition strongly inhibited N 2 fixation, nitrate did not affect nitrogenase in whole cells unless it was first reduced to nitrite. After 2.5 h of nitrite treatment, nitrogenase activity recovered upon nitrite elimination in a process independent from de novo protein synthesis. A similar nitrate inhibition of nitrogenase was found in a Rhodopseudomonas sphaeroides strain able to reduce nitrate. In contrast, nitrogenase of other strains incapable of reducing nitrate was unaffected by nitrate, but immediately inhibited by nitrite. The effect of nitrate on nitrogenase in Rp. capsulata E1F1 is then interpreted in terms of a short‐term inhibition by nitrite.