Strategies for increased ammonium production in free-living or plant associated nitrogen fixing bacteria
Strategies for increased ammonium production in free-living or plant associated nitrogen fixing bacteria
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DOI：
10.1023/a:1004268526162
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发表时间：
1997-07-01
期刊：
影响因子：
4.9
通讯作者：
Kennedy, C
中科院分区：
文献类型：
--
作者：
Colnaghi, R;Green, A;Kennedy, C
在固氮细菌中实现铵排泄所考虑和研究的策略包括1）抑制铵同化和2）干扰铵抑制固氮酶合成或活性的机制。固氮的这些方面已经在固氮变形菌和蓝藻中得到了最好的研究，本文对前者的研究进行了综述。谷氨酰胺合成酶（GS）的铵同化作用可以通过用抑制GS活性的化学物质处理细菌来减少或阻止，并且在某些固氮生物中，这种处理导致高达15 mM的铵排泄到液体生长培养基中。此外，通过选择GS抑制剂抗性分离的GS活性改变的突变体通常排泄铵。在变形菌中，铵抑制固氮酶活性和/或合成，后者通过阻止NifA的活性或表达，NifA是其他nif基因表达所需的转录激活因子。在棕色固氮菌中，铵抑制NifA活性但不抑制其合成; NifL通过直接与NifA相互作用导致其失活来介导这种效应。在nifL插入突变体中，NifA是组成型活性的，并且在固氮过程中分泌高达10 mM的铵。GlnD插入/缺失突变不能在A. vinelandii野生型，但在产生组成型活性GS（不能被腺苷酰化）的突变体中是稳定和可行的。这证实了GlnD是A. vinelandii。此外，稳定的glnD突变体是Nif（-），也支持先前的结论，即GlnD参与介导NifL/NifA相互作用。讨论和比较了铵离子抑制其他固氮生物NifA合成或活性的机理。
Strategies considered and studied for achieving ammonium excretion in nitrogen fixing bacteria include 1) inhibition of ammonium assimilation and 2) interference with the mechanisms by which ammonium inhibits either nitrogenase synthesis or activity. These aspects of nitrogen fixation have been best studied in diazotrophic Proteobacteria and Cyanobacteria and those of the former are reviewed in this paper. Ammonium assimilation by glutamine synthetase (GS) can be diminished or prevented by treatment of bacteria with chemicals that inhibit GS activity and in some diazotrophs, such treatment results in excretion of up to 15mM ammonium into liquid growth medium. Also, mutants with altered GS activity, isolated by selection for resistance to GS inhibitors, often excrete ammonium. In Proteobacteria, ammonium inhibits nitrogenase activity and/or synthesis, the latter by preventing activity or expression of NifA, a transcriptional activator required for expression of other nif genes. In Azotobacter vinelandii, ammonium inhibits NifA activity but not its synthesis; NifL mediates this effect by interacting directly with NifA causing its inactivation. In nifL insertion mutants, NifA is constitutively active and up to 10 mM ammonium is excreted during nitrogen fixation. GlnD insertion/deletion mutations are unable to be stably maintained in A. vinelandii wild type but are stable and viable in a mutant that produces constitutively active GS (cannot be adenylylated). This confirms the hypothesis that GlnD is required for activity of GS, an essential enzyme in A. vinelandii. In addition, the stable glnD mutants are Nif(-), supporting also the previous conclusion that GlnD is involved in mediating NifL/NifA interaction. Mechanisms of inhibition of synthesis or activity of NifA by ammonium in other diazotrophs are discussed and compared.