N_2-fixation in marine cyanophyte Trichodesmium cultured under artificial conditions

海洋浮游蓝藻，束毛藻属，在有氧条件下能在光照下固定N_2。本文对束毛藻在有氧条件下保持固氮活性的机理进行了研究。研究结果和进一步的看法如下：1.束毛藻固氮至少在两个不同的水平上受到调控，即固氮酶的生物合成和固氮酶Fe蛋白的翻译后修饰。2.免疫电镜观察表明，束毛藻没有形成蓝藻有氧固氮的特殊细胞（异形胞）。几乎所有的细胞都含有固氮酶。3.通过对固氮酶Fe蛋白结构基因（nifH）的序列测定，发现束毛藻固氮酶Fe蛋白一级结构与其它固氮生物具有很高的同源性。固氮酶铁钼蛋白（nif D，H）结构基因的测序正在进行中。4.束毛藻的固氮活性是由光照维持的。当细胞从光照转移到黑暗中时，活性突然丧失。固氮酶的铁蛋白在黑暗中变为无活性形式，由无活性形式转变为活性形式需要光照。蛋白质合成抑制剂通过光抑制活化过程。这些结果表明：（1）固氮酶活性的维持不仅需要光来提供光合作用所需的ATP和/或还原剂，而且还需要光来维持铁蛋白的活性形式;（2）在固氮酶的活化过程中，迅速翻转的蛋白质因子起作用。蛋白质因子的测定和光对固氮酶转录和翻译水平的影响的进一步分析是必需的。

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.