Biological nitrogen fixation is catalyzed by the molybdenum (Mo), vanadium (V) and iron (Fe)-only nitrogenase metalloenzymes. Studies with purified enzymes have found that the 'alternative' V- and Fe-nitrogenases generally reduce N2 more slowly and produce more byproduct H2 than the Mo-nitrogenase, leading to an assumption that their usage results in slower growth. Here we show that, in the metabolically versatile photoheterotroph Rhodopseudomonas palustris, the type of carbon substrate influences the relative rates of diazotrophic growth based on different nitrogenase isoforms. The V-nitrogenase supports growth as fast as the Mo-nitrogenase on acetate but not on the more oxidized substrate succinate. Our data suggest that this is due to insufficient electron flux to the V-nitrogenase isoform on succinate compared with acetate. Despite slightly faster growth based on the V-nitrogenase on acetate, the wild-type strain uses exclusively the Mo-nitrogenase on both carbon substrates. Notably, the differences in H2 :N2 stoichiometry by alternative nitrogenases (~1.5 for V-nitrogenase, ~4-7 for Fe-nitrogenase) and Mo-nitrogenase (~1) measured here are lower than prior in vitro estimates. These results indicate that the metabolic costs of V-based nitrogen fixation could be less significant for growth than previously assumed, helping explain why alternative nitrogenase genes persist in diverse diazotroph lineages and are broadly distributed in the environment.

中文翻译：

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碳底物使用 Mo-、V-或 Fe-固氮酶重新排列细菌的相对生长。

生物固氮由钼 (Mo)、钒 (V) 和铁 (Fe) 唯一的固氮酶金属酶催化。对纯化酶的研究发现，“替代”V-和 Fe-固氮酶通常比 Mo-固氮酶更慢地还原 N2 并产生更多的副产物 H2，从而假设它们的使用会导致生长缓慢。在这里，我们表明，在代谢多功能的光异养型沼泽红假单胞菌中，碳底物的类型会影响基于不同固氮酶异构体的固氮生长的相对速率。V-固氮酶在乙酸盐上的生长速度与 Mo-固氮酶一样快，但在氧化程度更高的底物琥珀酸盐上则不行。我们的数据表明，这是由于与醋酸盐相比，琥珀酸盐上的 V-固氮酶异构体的电子通量不足。尽管基于乙酸盐上的 V-固氮酶的生长稍快，但野生型菌株仅在两种碳底物上使用 Mo-固氮酶。值得注意的是，此处测量的替代固氮酶（V-固氮酶约为 1.5，铁固氮酶约为 4-7）和钼固氮酶（~1）的 H2：N2 化学计量差异低于先前的体外估计值。这些结果表明，基于 V 的固氮的代谢成本对生长的影响可能不如先前假设的那么重要，这有助于解释为什么替代固氮酶基因在不同的固氮菌谱系中持续存在并广泛分布在环境中。V-固氮酶为 5，Fe-固氮酶为 ~4-7) 和 Mo-固氮酶 (~1) 低于之前的体外估计值。这些结果表明，基于 V 的固氮的代谢成本对生长的影响可能不如先前假设的那么重要，这有助于解释为什么替代固氮酶基因在不同的固氮菌谱系中持续存在并广泛分布在环境中。V-固氮酶为 5，Fe-固氮酶为 ~4-7) 和 Mo-固氮酶 (~1) 低于之前的体外估计值。这些结果表明，基于 V 的固氮的代谢成本对生长的影响可能不如先前假设的那么重要，这有助于解释为什么替代固氮酶基因在不同的固氮菌谱系中持续存在并广泛分布在环境中。