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J. Biol. Chem., Vol. 283, Issue 43, 28842-28851, October 24, 2008

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Arabidopsis Phosphomannose Isomerase 1, but Not Phosphomannose Isomerase 2, Is Essential for Ascorbic Acid Biosynthesis^*

Takanori Maruta, Miki Yonemitsu, Yukinori Yabuta, Masahiro Tamoi, Takahiro Ishikawa^¶, and Shigeru Shigeoka¹

From the Department of Advanced Bioscience, Faculty of Agriculture, Kinki University, 3327-204 Nakamachi, Nara 631-8505, School of Agricultural, Biological and Environmental Sciences, Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori 680-8550, and ^¶Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan

We studied molecular and functional properties of Arabidopsis phosphomannose isomerase isoenzymes (PMI1 and PMI2) that catalyze reversible isomerization between D-fructose 6-phosphate and D-mannose 6-phosphate (Man-6P). The apparent K_m and V_max values for Man-6P of purified recombinant PMI1 were 41.3 ± 4.2 µM and 1.89 µmol/min/mg protein, respectively, whereas those of purified recombinant PMI2 were 372 ± 13 µM and 22.5 µmol/min/mg protein, respectively. Both PMI1 and PMI2 were inhibited by incubation with EDTA, Zn²⁺, Cd²⁺, and L-ascorbic acid (AsA). Arabidopsis PMI1 protein was constitutively expressed in both vegetative and reproductive organs under normal growth conditions, whereas the PMI2 protein was not expressed in any organs under light. The induction of PMI1 expression and an increase in the AsA level were observed in leaves under continuous light, whereas the induction of PMI2 expression and a decrease in the AsA level were observed under long term darkness. PMI1 showed a diurnal expression pattern in parallel with the total PMI activity and the total AsA content in leaves. Moreover, a reduction of PMI1 expression through RNA interference resulted in a substantial decrease in the total AsA content of leaves of knockdown PMI1 plants, whereas the complete inhibition of PMI2 expression did not affect the total AsA levels in leaves of knock-out PMI2 plants. Consequently, this study improves our understanding of the molecular and functional properties of Arabidopsis PMI isoenzymes and provides genetic evidence of the involvement of PMI1, but not PMI2, in the biosynthesis of AsA in Arabidopsis plants.

Received for publication, July 21, 2008 , and in revised form, August 25, 2008.

^* This work was supported by Scientific Research for Plant Graduate Students from the Nara Institute of Science and Technology (to T. M.), Grant-in-aid for Scientific Research 19208031 from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the "Academic Frontier" Project for Private Universities matching fund subsidy from Ministry of Education, Culture, Sports, Science and Technology 2004–2008 (to S. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3.

¹ To whom correspondence should be addressed. Tel./Fax: 81-742-43-8083; E-mail: shigeoka{at}nara.kindai.ac.jp.

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