Department of Biology, University of Oxford, OX1 3RB, Oxford, UK.
John Innes Centre, NR4 7UH, Norwich, UK.
Microbiology (Reading). 2024 Jul;170(7). doi: 10.1099/mic.0.001471.
aspartate aminotransferase (AatA) mutants show drastically reduced symbiotic nitrogen fixation in legume nodules. Whilst AatA reversibly transaminates the two major amino-donor compounds aspartate and glutamate, the reason for the lack of N fixation in the mutant has remained unclear. During our investigations into the role of AatA, we found that it catalyses an additional transamination reaction between aspartate and pyruvate, forming alanine. This secondary reaction runs at around 60 % of the canonical aspartate transaminase reaction rate and connects alanine biosynthesis to glutamate via aspartate. This may explain the lack of any glutamate-pyruvate transaminase activity in , which is common in eukaryotic and many prokaryotic genomes. However, the aspartate-to-pyruvate transaminase reaction is not needed for N fixation in legume nodules. Consequently, we show that aspartate degradation is required for N fixation, rather than biosynthetic transamination to form an amino acid. Hence, the enzyme aspartase, which catalyses the breakdown of aspartate to fumarate and ammonia, suppressed an AatA mutant and restored N fixation in pea nodules.
天冬氨酸氨基转移酶（AatA）突变体在豆科植物根瘤中表现出明显降低的共生固氮作用。虽然 AatA 可逆地转氨基作用于两种主要的氨基供体化合物天冬氨酸和谷氨酸，但突变体中缺乏固氮的原因仍不清楚。在我们对 AatA 作用的研究中，我们发现它还催化天冬氨酸和丙酮酸之间的另一种转氨基反应，形成丙氨酸。这个次要反应的速率约为经典天冬氨酸转氨酶反应的 60%，并通过天冬氨酸将丙氨酸生物合成与谷氨酸连接起来。这可能解释了为什么在中缺乏任何谷氨酸-丙酮酸转氨酶活性，这在真核生物和许多原核生物基因组中很常见。然而，对于豆科植物根瘤中的固氮作用，天冬氨酸到丙酮酸的转氨基反应并不是必需的。因此，我们表明，天冬氨酸的降解对于固氮作用是必需的，而不是形成氨基酸的合成转氨基作用。因此，催化天冬氨酸分解为富马酸和氨的天冬氨酸酶抑制 AatA 突变体并恢复豌豆根瘤中的固氮作用。
