N_2-fixation in marine cyanophyte Trichodesmium cultured under artificial conditions

海洋，远洋蓝藻，trichodesium，能够在光照下的好氧条件下固定N_2。研究了在好氧条件下木霉维持固氮活性的机理。结果和进一步的看法如下：木霉的n_2固定至少受到两个不同水平的调控，即氮酶的生物合成和氮酶铁蛋白的翻译后修饰。2. 免疫电镜检查显示，赤霉病菌没有形成蓝藻中作为需氧氮固定位点的特殊细胞（异囊）。几乎所有的细胞都含有氮酶。3. 通过对结构基因（nif H）的测序，发现木霉菌与其他固氮生物的氮酶铁蛋白一级结构高度同源。氮酶的femo蛋白（nif D，H）结构基因的测序正在进行中。4. 光维持木霉的n_2固定活性。当细胞从光照环境转移到黑暗环境时，活动突然消失。氮酶铁蛋白在黑暗条件下变为失活状态，光照条件下由失活状态变为活性状态。蛋白质合成抑制剂抑制光的激活过程。这些结果表明：(1)维持氮酶活性不仅需要光来通过光合作用提供ATP和/或还原剂，还需要光来保持铁蛋白的活性形式；(2)在激活过程中快速翻转的蛋白质因子(s)起作用。需要测定蛋白质因子和进一步分析光对氮酶转录和翻译水平的影响。

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.