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Journal of Bacteriology, July 2006, p. 4635-4645, Vol. 188, No. 13
0021-9193/06/$08.00+0     doi:10.1128/JB.01999-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Flavin Adenine Dinucleotide-Dependent 4-Phospho-D-Erythronate Dehydrogenase Is Responsible for the 4-Phosphohydroxy-L-Threonine Pathway in Vitamin B6 Biosynthesis in Sinorhizobium meliloti

Masaaki Tazoe,* Keiko Ichikawa,{dagger} and Tatsuo Hoshino{ddagger}

Department of Applied Microbiology, Nippon Roche Research Center, Kamakura, Kanagawa 247-8530, Japan

Received 31 December 2005/ Accepted 10 April 2006

The vitamin B6 biosynthetic pathway in Sinorhizobium meliloti is similar to that in Escherichia coli K-12; in both organisms this pathway includes condensation of two intermediates, 1-deoxy-D-xylulose 5-phosphate and 4-phosphohydroxy-L-threonine (4PHT). Here, we report cloning of a gene designated pdxR that functionally corresponds to the pdxB gene of E. coli and encodes a dye-linked flavin adenine dinucleotide-dependent 4-phospho-D-erythronate (4PE) dehydrogenase. This enzyme catalyzes the oxidation of 4PE to 3-hydroxy-4-phosphohydroxy-{alpha}-ketobutyrate and is clearly different in terms of cofactor requirements from the pdxB gene product of E. coli, which is known to be an NAD-dependent enzyme. Previously, we revealed that in S. meliloti IFO 14782, 4PHT is synthesized from 4-hydroxy-L-threonine and that this synthesis starts with glycolaldehyde and glycine. However, in this study, we identified a second 4PHT pathway in S. meliloti that originates exclusively from glycolaldehyde (the major pathway). Based on the involvement of 4PE in the 4PHT pathway, the incorporation of different samples of 13C-labeled glycolaldehyde into pyridoxine molecules was examined using 13C nuclear magnetic resonance spectroscopy. On the basis of the spectral analyses, the synthesis of 4PHT from glycolaldehyde was hypothesized to involve the following steps: glycolaldehyde is sequentially metabolized to D-erythrulose, D-erythrulose 4-phosphate, and D-erythrose 4-phosphate by transketolase, kinase, and isomerase, respectively; and D-erythrose 4-phosphate is then converted to 4PHT by the conventional three-step pathway elucidated in E. coli, although the mechanism of action of the enzymes catalyzing the first two steps is different.

* Corresponding author. Mailing address: Mycology and Metabolic Diversity Research Center, Tamagawa University Research Institute, Machida, Tokyo 194-8610, Japan. Phone: 81-42-739-8682. Fax: 81-42-739-8669. E-mail: m.tazoe{at}lab.tamagawa.ac.jp.

{dagger} Present address: Research Planning and Coordination Department, Kamakura Research Laboratories, Chugai Pharmaceutical Co., Ltd., 200 Kajiwara, Kamakura, Kanagawa 247-8530, Japan.

{ddagger} Present address: Mycology and Metabolic Diversity Research Center, Tamagawa University Research Institute, Machida, Tokyo 194-8610, Japan.

Journal of Bacteriology, July 2006, p. 4635-4645, Vol. 188, No. 13
0021-9193/06/$08.00+0     doi:10.1128/JB.01999-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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