Mandon K, Kaminski P A, Elmerich C
Département des Biotechnologies, Institut Pasteur, Paris, France.
J Bacteriol. 1994 May;176(9):2560-8. doi: 10.1128/jb.176.9.2560-2568.1994.
The deduced amino acid sequences of four open reading frames identified upstream of the fixGHI region in Azorhizobium caulinodans are very similar to the putative terminal oxidase complex coded by the fixNOQP operons from Rhizobium meliloti and Bradyrhizobium japonicum. The expression of the A. caulinodans fixNOQP genes, which was maximal under microaerobiosis, was positively regulated by FixK and independent of NifA. In contrast to the Fix- phenotype of B. japonicum and R. meliloti fixN mutants, an A. caulinodans fixNO-deleted mutant strain retained 50% of the nitrogenase activity of the wild type in the symbiotic state. In addition, the nitrogenase activity was scarcely reduced under free-living conditions. Analysis of membrane fractions of A. caulinodans wild-type and mutant strains suggests that the fixNOQP region encodes two proteins with covalently bound hemes, tentatively assigned to fixO and fixP. Spectral analysis showed a large decrease in the c-type cytochrome content of the fixN mutant compared with the wild type. These results provide evidence for the involvement of FixNOQP proteins in a respiratory process. The partial impairment in nitrogen fixation of the fixN mutant in planta may be due to the activity of an alternative terminal oxidase compensating for the loss of the oxidase complex encoded by fixNOQP.
在茎瘤固氮根瘤菌fixGHI区域上游鉴定出的四个开放阅读框推导的氨基酸序列，与来自苜蓿根瘤菌和日本慢生根瘤菌的fixNOQP操纵子编码的假定末端氧化酶复合物非常相似。茎瘤固氮根瘤菌fixNOQP基因的表达在微需氧条件下最高，受FixK正调控且不依赖于NifA。与日本慢生根瘤菌和苜蓿根瘤菌fixN突变体的Fix-表型相反，茎瘤固氮根瘤菌fixNO缺失突变体菌株在共生状态下保留了野生型50%的固氮酶活性。此外，在自由生活条件下固氮酶活性几乎没有降低。对茎瘤固氮根瘤菌野生型和突变体菌株膜组分的分析表明，fixNOQP区域编码两种带有共价结合血红素的蛋白质，暂定为fixO和fixP。光谱分析表明，与野生型相比，fixN突变体的c型细胞色素含量大幅下降。这些结果为FixNOQP蛋白参与呼吸过程提供了证据。fixN突变体在植物中固氮的部分受损可能是由于替代末端氧化酶的活性补偿了fixNOQP编码的氧化酶复合物的缺失。
