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Front. Plant Sci. | doi: 10.3389/fpls.2018.00191

Chloroplast glutamine synthetase, the key regulator of nitrogen metabolism in wheat, performs its role by fine regulation of enzyme activity via negative cooperativity of its subunits

  • 1Department of Plant Biology, University of Szeged, Hungary
  • 2Doctoral School in Biology, Faculty of Science and Informatics, University of Szeged, Hungary
  • 3Cereal Research (Hungary), Hungary

Glutamine synthetase (GS) is of central interest as the main route of ammonia assimilation in plants, and as a connection point between the organic and inorganic worlds. Even though GS activity is critical for producing high yields of crop plants, the autoregulation of substrate consumption of wheat GS remained unknown until now. Here we show kinetic evidence, that the chloroplast localized GS isoform (GS2) of wheat (Triticum aestivum L. cv. Jubilejnaja-50) takes place at the carbon-nitrogen metabolic branch point, where it is a mediator, and its enzymatic activity is regulated in a negatively cooperative allosteric manner. We have discovered that GS2 activity is described by a tetraphasic kinetic curve in response to increasing levels of glutamate supply. We constructed a model that explains the kinetic properties of glutamate consumption and this unique allosteric behavior. We also studied the subunit composition of both wheat leaf GS isoenzymes by a combination of two dimensional gel electrophoresis and protein blotting. Both leaf isozymes have homogeneous subunit composition. Glutamate is both a substrate, and an allosteric regulator of the biosynthetic reaction. We have concluded on the basis of our results and previous reports, that wheat GS2 is probably a homooctamer, and that it processes its substrate in a well-regulated, concentration dependent way, as a result of its negatively cooperative, allosteric activity. Thus GS2 has a central role as a regulator between the nitrogen and the carbon cycles via maintaining glutamine-glutamate pool in the chloroplast on the level of substrates, in addition to its function in ammonia assimilation.

Keywords: Allosteric behavior, glutamine synthetase, Negative cooperativity, nitrogen metabolism, Triticum aestivum

Received: 28 Nov 2017; Accepted: 01 Feb 2018.

Edited by:

Fumiya Kurosaki, University of Toyama, Toyama, Japan

Reviewed by:

Yan Lu, Western Michigan University, United States
Takahiro Mori, ETH Zurich, Switzerland  

Copyright: © 2018 Németh, Nagy and Pécsváradi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Attila Pécsváradi, University of Szeged, Department of Plant Biology, Közép fasor 52., Szeged, 6726, Hungary, pecsvaradi@bio.u-szeged.hu