Zhang Yaoping, Wolfe David M, Pohlmann Edward L, Conrad Mary C, Roberts Gary P
Department of Bacteriology and the Center for the Study of Nitrogen Fixation, University of Wisconsin-Madison, Madison, WI 53706, USA.
Microbiology (Reading). 2006 Jul;152(Pt 7):2075-2089. doi: 10.1099/mic.0.28903-0.
The AmtB protein transports uncharged NH(3) into the cell, but it also interacts with the nitrogen regulatory protein P(II), which in turn regulates a variety of proteins involved in nitrogen fixation and utilization. Three P(II) homologues, GlnB, GlnK and GlnJ, have been identified in the photosynthetic bacterium Rhodospirillum rubrum, and they have roles in at least four overlapping and distinct functions, one of which is the post-translational regulation of nitrogenase activity. In R. rubrum, nitrogenase activity is tightly regulated in response to addition or energy depletion (shift to darkness), and this regulation is catalysed by the post-translational regulatory system encoded by draTG. Two amtB homologues, amtB(1) and amtB(2), have been identified in R. rubrum, and they are linked with glnJ and glnK, respectively. Mutants lacking AmtB(1) are defective in their response to both addition and darkness, while mutants lacking AmtB(2) show little effect on the regulation of nitrogenase activity. These responses to darkness and appear to involve different signal transduction pathways, and the poor response to darkness does not seem to be an indirect result of perturbation of internal pools of nitrogen. It is also shown that AmtB(1) is necessary to sequester detectable amounts GlnJ to the cell membrane. These results suggest that some element of the AmtB(1)-P(II) regulatory system senses energy deprivation and a consistent model for the integration of nitrogen, carbon and energy signals by P(II) is proposed. Other results demonstrate a degree of specificity in interaction of AmtB(1) with the different P(II) homologues in R. rubrum. Such interaction specificity might be important in explaining the way in which P(II) proteins regulate processes involved in nitrogen acquisition and utilization.
AmtB蛋白将不带电荷的NH₃转运到细胞内，但它也与氮调节蛋白P(II)相互作用，而P(II)反过来又调节多种参与固氮和氮利用的蛋白质。在光合细菌红螺菌中已鉴定出三种P(II)同源物，即GlnB、GlnK和GlnJ，它们至少在四种重叠且不同的功能中发挥作用，其中之一是对固氮酶活性的翻译后调节。在红螺菌中，固氮酶活性会根据铵的添加或能量耗尽（转入黑暗）而受到严格调节，这种调节由draTG编码的翻译后调节系统催化。在红螺菌中已鉴定出两种amtB同源物，即amtB(1)和amtB(2)，它们分别与glnJ和glnK相关联。缺乏AmtB(1)的突变体对铵添加和黑暗的反应均有缺陷，而缺乏AmtB(2)的突变体对固氮酶活性的调节几乎没有影响。这些对黑暗的反应似乎涉及不同的信号转导途径，对黑暗反应不佳似乎并非氮内部库受到干扰的间接结果。研究还表明，AmtB(1)对于将可检测量的GlnJ隔离到细胞膜是必需的。这些结果表明，AmtB(1)-P(II)调节系统的某些元件可感知能量剥夺，并提出了一个P(II)整合氮、碳和能量信号的一致模型。其他结果表明，红螺菌中AmtB(1)与不同P(II)同源物之间的相互作用具有一定程度的特异性。这种相互作用特异性对于解释P(II)蛋白调节氮获取和利用相关过程的方式可能很重要。
