Aspartate aminotransferase of Rhizobium leguminosarum has extended substrate specificity and metabolizes aspartate to enable N2 fixation in pea nodules

Author:

Ledermann Raphael1ORCID,Bourdès Alexandre23,Schuller Marion4,Jorrin Beatriz1,Ahel Ivan4ORCID,Poole Philip Simon231

Affiliation:

1. Department of Biology, University of Oxford, OX1 3RB, Oxford, UK

2. School of Animal and Microbial Sciences, University of Reading, RG6 6AJ, Reading, UK

3. John Innes Centre, NR4 7UH, Norwich, UK

4. Sir William Dunn School of Pathology, University of Oxford, OX1 3RE, Oxford, UK

Abstract

Rhizobium leguminosarum 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 N2 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 R. leguminosarum, which is common in eukaryotic and many prokaryotic genomes. However, the aspartate-to-pyruvate transaminase reaction is not needed for N2 fixation in legume nodules. Consequently, we show that aspartate degradation is required for N2 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 N2 fixation in pea nodules.

Funder

Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung

Biotechnology and Biological Sciences Research Council

Publisher

Microbiology Society

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