The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum is critical to the agro-industrial production of soybean because it enables the production of high yields of soybeans with little use of nitrogenous fertilizers. The FixL and FixJ two-component system (TCS) of this bacterium ensures that nitrogen fixation is only stimulated under conditions of low oxygen. When it is not bound to oxygen, the histidine kinase FixL undergoes autophosphorylation and transfers phosphate from adenosine triphosphate (ATP) to the response regulator FixJ, which, in turn, stimulates the expression of genes required for nitrogen fixation. We purified full-length B. japonicum FixL and FixJ proteins and defined their structures individually and in complex using small-angle x-ray scattering, crystallographic, and in silico modeling techniques. Comparison of active and inactive forms of FixL suggests that intramolecular signal transduction is driven by local changes in the sensor domain and in the coiled-coil region connecting the sensor and histidine kinase domains. We also found that FixJ exhibits conformational plasticity not only in the monomeric state but also in tetrameric complexes with FixL during phosphotransfer. This structural characterization of a complete TCS contributes both a mechanistic and evolutionary understanding to TCS signal relay, specifically in the context of the control of nitrogen fixation in root nodules.

共生固氮细菌对日本大豆的农业生产至关重要，因为它可以在不使用氮肥的情况下生产出高产的大豆。该细菌的FixL和FixJ两组分系统（TCS）确保仅在低氧条件下才能刺激固氮。当它不与氧结合时，组氨酸激酶FixL会进行自磷酸化，并将磷酸酯从三磷酸腺苷（ATP）转移到响应调节器FixJ，后者进而刺激固氮所需的基因的表达。我们纯化了全长的日本血吸虫FixL和FixJ蛋白，并使用小角度X射线散射，晶体学和计算机模拟技术分别和复杂地定义了它们的结构。FixL的活性形式和非活性形式的比较表明，分子内信号转导是由传感器结构域以及连接传感器和组氨酸激酶结构域的卷曲螺旋区域中的局部变化驱动的。我们还发现，FixJ在磷酸转移过程中不仅在单体状态而且在与FixL的四聚体复合物中均显示出构象可塑性。完整TCS的这种结构特征有助于TCS信号中继在机理和进化上的理解，特别是在根瘤中固氮控制的背景下。