Glutamine synthetase in legumes: recent advances in enzyme structure and functional genomics

Abstract

Glutamine synthetase (GS) is the key enzyme involved in the assimilation of ammonia derived either from nitrate reduction, N(2) fixation, photorespiration or asparagine breakdown. A small gene family is encoding for different cytosolic (GS1) or plastidic (GS2) isoforms in legumes. We summarize here the recent advances carried out concerning the quaternary structure of GS, as well as the functional relationship existing between GS2 and processes such as nodulation, photorespiration and water stress, in this latter case by means of proline production. Functional genomic analysis using GS2-minus mutant reveals the key role of GS2 in the metabolic control of the plants and, more particularly, in carbon metabolism.

Keywords: Lotus japonicus; functional genomics; glutamine synthetase; nitrogen metabolism.

Figure 1

Nitrogen utilization in legume plants. Simple arrows represent single step reaction, while consecutive arrows represent multiple-step reactions. GS₁, cytosolic glutamine synthetase; GS₂, plastidic glutamine synthetase; NR, nitrate reductase; NiR, nitrite reductase.

Figure 2

Enzymes involved in glutamine metabolism. GS, glutamine synthetase; GOGAT, glutamate synthase; AAT, aspartate aminotransferase; AS, asparagine synthetase; NSE, asparaginase.

Figure 3

Differences in the amount of inter-ring subunit interactions of type I and type II GS. The figure shows in orange the main contact region among subunits existing in Salmonella typhimurium GS (type I GS) compared to GS1a from maize (type II GS) as drawn in a lateral view from their corresponding three dimensional structures.

Figure 4

Co-expression analysis of plastidic GS. An overview of general metabolism was created using MapMan. Blue squares represent genes that are positively co-expressed (Pearson distance <0.3) with plastidic GS. The intensity of the blue color is proportional to the degree of correlation between a certain gene and the plastidic GS gene.