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Plant and Cell Physiology Advance Access originally published online on December 7, 2005
Plant and Cell Physiology 2006 47(2):299-303; doi:10.1093/pcp/pci238
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Short Communication

Gln49 and Ser174 Residues Play Critical Roles in Determining the Catalytic Efficiencies of Plant Glutamine Synthetase

Keiki Ishiyama1, Eri Inoue1, Tomoyuki Yamaya2 and Hideki Takahashi1,*

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

* Corresponding author: E-mail, hideki{at}riken.jp; Fax, +81-45-503-9650.

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 K m values for ammonium. In contrast, replacement of Gln49 and Ser174 by lysine and alanine, respectively, was detrimental to glutamine synthetic activities in GLN1;4. The results suggested that Gln49 and Ser174 in the high-affinity GS1 isoenzymes are interchangeable with Lys49 and Ala174 in the low-affinity variants at the corresponding positions.

(Received October 9, 2005; Accepted November 29, 2005)
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