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Plant and Cell Physiology Advance Access published online on December 7, 2005
Plant and Cell Physiology, doi:10.1093/pcp/pci238
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Plant and Cell Physiology 2005 © The Japanese Society of Plant Physiologists (JSPP); all rights reserved.
Received October 9, 2005
Accepted November 29, 2005

Short Communication

Gln-49 and Ser-174 Residues Play Critical Roles in Determining the Catalytic Efficiencies of Plant Glutamine Synthetase

Keiki Ishiyama 1, Eri Inoue 1, Tomoyuki Yamaya 2, and Hideki Takahashi 1 *

1 RIKEN Plant Science Center, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan
2 Graduate School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555, Japan

* To whom correspondence should be addressed.
Hideki Takahashi, E-mail: hideki{at}riken.jp


  Abstract

Two essential residues playing critical roles in determining the substrate specificities of cytosolic glutamine synthetase (GS1) have been identified from the alignment of high-affinity (GLN1;1 and GLN1;4) and low-affinity (GLN1;2 and GLN1;3) GS1 isoenzymes in Arabidopsis, and confirmed by site-directed mutagenesis. The results indicated that either K49Q or A174S mutation is sufficient to increase the catalytic efficiencies of GLN1;3 by decreasing its Km values for ammonium. By contrast, replacement of Gln-49 and Ser-174 to Lys and Ala, respectively, was detrimental to Gln synthetic activities in GLN1;4. The results suggested that Gln-49 and Ser-174 in the high-affinity GS1 isoenzymes are interchangeable with Lys-49 and Ala-174 in the low-affinity variants at the corresponding positions.

Keywords: Ammonium assimilation; Arabidopsis thaliana; Catalytic efficiency; Glutamine synthetase; Nitrogen metabolism; Site-directed mutagenesis.
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