Noonan B, Motherway M, O'Gara F
Department of Microbiology, University College, Cork, Republic of Ireland.
Mol Gen Genet. 1992 Sep;234(3):423-8. doi: 10.1007/BF00538701.
The expression under microaerobic conditions of the Rhizobium meliloti nifA and consequently the nifHDK genes was found to be negatively regulated by ammonia and nitrate. Assimilation of the ammonia to glutamate and glutamine is not required for this regulation to occur. This indicates that ammonia itself, and not a product of its metabolism, may be regulating nif expression. Unlike the situation in Klebsiella pneumoniae, NtrC is apparently not involved in mediating the ammonia effect on nifA expression in R. meliloti. Neither does the fixK gene product, which is known to regulate nifA in R. meliloti, appear to be involved in mediating the ammonia effect. The regulation of nifA by ammonia is shown to be mediated through the FixL protein. A truncated fixJ gene, the product of which has been shown to induce nifA expression irrespective of the oxygen status of the cell, also circumvented the repressive effect of ammonia on nifA expression. This suggests that the ammonia effect is mediated through the FixLJ regulatory cascade. Interestingly no effect of ammonia on fixK expression was observed.
已发现，在微需氧条件下，苜蓿根瘤菌的nifA基因以及随后的nifHDK基因的表达受到氨和硝酸盐的负调控。这种调控的发生并不需要将氨同化为谷氨酸和谷氨酰胺。这表明，是氨本身而非其代谢产物可能在调控固氮基因的表达。与肺炎克雷伯菌的情况不同，NtrC显然不参与介导氨对苜蓿根瘤菌nifA基因表达的影响。已知在苜蓿根瘤菌中调控nifA基因的fixK基因产物似乎也不参与介导氨的影响。研究表明，氨对nifA基因的调控是通过FixL蛋白介导的。一个截短的fixJ基因，其产物已被证明无论细胞的氧状态如何都能诱导nifA基因的表达，它也能规避氨对nifA基因表达的抑制作用。这表明氨的影响是通过FixLJ调控级联介导的。有趣的是，未观察到氨对fixK基因表达有影响。
