A bifunctional epimerase‐reductase acts downstream of the MUR1 gene product and completes the de novo synthesis of GDP‐L‐fucose in Arabidopsis

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Abstract
L‐Fucose is a monosaccharide found as a component of glycoproteins and cell wall polysaccharides in higher plants. The MUR1 gene of Arabidopsis thaliana encodes a GDP‐ d‐mannose 4,6‐dehydratase catalyzing the first step in the de novo synthesis of GDP‐ l‐fucose from GDP‐ d‐mannose ( Bonin et al. 1997 , Proc. Natl Acad. Sci. USA, 94, 2085–2090). Plant genes encoding the subsequent steps in l‐fucose synthesis (3,5‐epimerization and 4‐reduction) have not been described previously. Based on sequence similarities to a bacterial gene involved in capsule synthesis we have cloned a gene from Arabidopsis, now designated GER1, which encodes a bifunctional 3,5‐epimerase‐4‐reductase in l‐fucose synthesis. The combined action of the MUR1 and GER1 gene products converts GDP‐ d‐mannose to GDP‐ l‐fucose in vitro demonstrating that this entire nucleotide–sugar interconversion pathway could be reconstituted using plant genes expressed in Escherichia coli. In vitro assays indicated that the GER1 protein does not act as a GDP‐ d‐mannose 3,5‐epimerase, an enzymatic activity involved in the de novo synthesis of GDP‐ l‐galactose and l‐ascorbic acid. Similarly, l‐ascorbate levels in GER1 antisense plants were unchanged indicating that GDP‐ d‐mannose 3,5‐epimerase is encoded by a separate gene.