Iron Uptake and Metabolism in the Bradyrhizobium/Soybean Symbiosis

含铁蛋白在细菌和它们各自的植物宿主之间的固氮共生体中占有重要地位。虽然铁在土壤中含量丰富，但在好氧条件下，由于铁在生物pH下的低溶解度，铁的获取是有问题的。由于铁的获取属于经济上重要的共生生物，因此对它的研究旨在回答三个问题：1)根瘤细胞如何作为自由生活的微生物获取铁，它们必须与其他土壤居民竞争这种营养；2)根瘤菌如何作为共生体获取铁；3)铁是否调节基因的表达？关于铁载体柠檬酸盐、铬铁和浊浊液作为铁源的研究仍在继续，此外还在研究铁对铁摄取相关基因的调控。本研究：1)进一步从生物化学的角度研究日本血吸虫自生细胞和共生细胞对铁的获取；2)鉴定与铁获取相关的基因，包括编码铁诱导的外膜蛋白的基因；3)研究铁对基因表达的调控。随着世界对蛋白质需求的增加和氮肥生产成本的增加，生物固氮引起了人们的广泛兴趣。尽管在世界温带地区，固氮慢生根瘤菌/豆科植物共生体为农业提供了大量的固定氮，但对共生体的许多方面仍知之甚少。铁在动物/病原体相互作用中的重要性现在已经得到了很好的证实；这种金属在固氮根瘤菌/豆科共生中的作用才刚刚开始探索。这项研究为更好地理解铁在调节共生相互作用中的作用提供了依据。这样的理解阐明了根瘤菌如何在土壤中生存，以及它们如何与植物宿主相互作用，建立有效的共生关系。这对于设计更好的共生关系以及最终将共生关系扩展到其他植物寄主是至关重要的。

Iron-containing proteins figure prominently in the nitrogen- fixing symbioses between bacteria and their respective plant hosts. Although iron is abundant in the soil, the acquisition of iron is problematic due to its low solubility at biological pH under aerobic conditions. The study of iron acquisition as it pertains to economically important symbioses is directed at answering three questions: 1) how do rhizobial cells acquire iron as free-living microorganisms where they must compete for this nutrient with other soil inhabitants 2) how do rhizobia acquire iron as symbionts and 3) does iron regulate gene expression? Studies are continuing on the utilization of the siderophores citrate, ferrichrome and rhodoturulate as iron sources by B. japonicum as well as on the iron regulation of genes involved in iron uptake. This research: 1) further characterizes biochemically the acquisition of iron by both free-living and symbiotic cells of B. japonicum, 2) identifies and characterizes genes involved in iron acquisition, including those which encode iron-inducible outer membrane proteins, and 3) investigates the control of gene expression by iron. The increasing world demand for protein and the increasing expense of producing nitrogen fertilizer has resulted in a widespread interest in biological nitrogen fixation. Although the nitrogen-fixing Bradyrhizobium/legume symbioses supply much of the fixed nitrogen for agriculture in the temperate regions of the world, many aspects of the symbioses are still poorly understood. The importance of iron in animal/pathogen interactions is now well established; the role of this metal in the nitrogen-fixing rhizobial/legume symbioses is just beginning to be explored. The research proposed here provides for a better understanding of the role of iron in regulating the symbiotic interaction. Such an understanding elucidates how rhizobia survive in the soil as well as how they interact with their plant host to establish an effective symbiosis. This is essential to any efforts to engineer a better symbiosis and for the ultimate extension of the symbiosis to other plant hosts.