N_2-fixation in marine cyanophyte Trichodesmium cultured under artificial conditions

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Marine, pelagic cyanophyte, Trichodesmium, able to fix N_2 under aerobic conditions in the light. Mechanisms for the maintenance of N_2-fixation activity in Trichodesmium under aerobic conditions were studied. Results and further view are as follow : 1. N_2-fixation of Trichodesmium is regulated at least two different levels, biosynthesis of nitrogenase and post-translational modification of the Fe-protein of nitrogenase. 2. Examination by immuno-electron microscopy showed that Trichodesmium did not develop the special cells (heterocysts) which were the site of aerobic N_2-fixation in cyanophyte. Almost all cells contain nitrogenase. 3. The high homology of the primary structure of the Fe-protein of nitrogenase between Trichodesmium and other N_2-fixing organisms was found by sequencing of the structure gene (nif H). Sequencing of the structure genes of the FeMo-protein of nitrogenase (nif D,H) is under progress. 4. N_2-fixation activity of Trichodesmium is maintained by the light. Activity was abruptly lost when cells were transferred from light to the dark. The Fe-protein of nitrogenase became inactive form in the dark, and light was required to change from inactive to active form. Protein synthesis inhibitors inhibit the activation process by light. These results suggest that (1)light is required to maintain the nitrogenase activity not only to supply ATP and/or reductant by photosynthesis but also keeping the Fe-protein active form, and that (2)proteinous factor(s) which turns over rapidly acts during the activation process. Determination of proteinous factors(s) and further analysis of the effect of light on transcriptional and translational levels of nitrogenase are required.