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. 1989 Aug;90(4):1622–1629. doi: 10.1104/pp.90.4.1622

Aspartate Aminotransferase in Alfalfa Root Nodules 1

I. Purification and Partial Characterization

Stephen M Griffith 1,2,2, Carroll P Vance 1,2
PMCID: PMC1061933  PMID: 16666973

Abstract

Aspartate aminotransferase (l-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1 [AAT]), a key enzyme in the assimilation of C and N compounds, was purified from the cytosol of alfalfa (Medicago sativa L.) root nodules. Isoforms that increased during nodule development, AAT-2a, AAT-2b, and AAT-2c, were purified greater than 447-fold to apparent homogeneity, and high titer polyclonal antibodies were produced. The native molecular weight of the AAT-2 isoforms was approximately 80 kilodatons with a subunit molecular weight of 40 kilodatons, indicating that the holoenzymes are dimers. The AAT-2 isoforms comprised approximately 0.4% of the total soluble nodule protein. The AAT specific activity was measured in leaf, stem, root, and nodule organs, and zymograms of each were compared. Enzyme activity was 4- to 37-fold greater in effective (nitrogen fixing) nodules than in leaves, stems, and roots. Effective nodule AAT-specific activity was 3- to 8-fold greater than that of plant-controlled ineffective nodules. No differences in Km were observed between AAT-1 and AAT-2. Antibodies raised against AAT-2 were more selective against AAT-2 than AAT-1. Evidence obtained from zymograms suggests that the expression of alfalfa nodule AAT is controlled at two different gene loci, AAT-1 and AAT-2, resulting in different dimeric isoforms.

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Selected References

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