Xyloglucans are the principal glycans that interlace cellulose microfibrils in most flowering plants. The mur3 mutant of Arabidopsis contains a severely altered structure of this polysaccharide because of the absence of a conserved alpha-L-fucosyl-(1-->2)-beta-D-galactosyl side chain and excessive galactosylation at an alternative xylose residue. Despite this severe structural alteration, mur3 plants were phenotypically normal and exhibited tensile strength in their inflorescence stems comparable to that of wild-type plants. The MUR3 gene was cloned positionally and shown to encode a xyloglucan galactosyltransferase that acts specifically on the third xylose residue within the XXXG core structure of xyloglucan. MUR3 belongs to a large family of type-II membrane proteins that is evolutionarily conserved among higher plants. The enzyme shows sequence similarities to the glucuronosyltransferase domain of exostosins, a class of animal glycosyltransferases that catalyze the synthesis of heparan sulfate, a glycosaminoglycan with numerous roles in cell differentiation and development. This finding suggests that components of the plant cell wall and of the animal extracellular matrix are synthesized by evolutionarily related enzymes even though the structures of the corresponding polysaccharides are entirely different from each other.

Cell walls of the Arabidopsis mutant mur2 contain less than 2% of the wild-type amount of fucosylated xyloglucan because of a point mutation in the fucosyltransferase AtFUT1 . The mur2 mutation eliminates xyloglucan fucosylation in all major plant organs, indicating that Arabidopsis thaliana fucosyltransferase 1 (AtFUT1) accounts for all of the xyloglucan fucosyltransferase activity in Arabidopsis . Despite this alteration in structure, mur2 plants show a normal growth habit and wall strength. In contrast, Arabidopsis mur1 mutants that are defective in the de novo synthesis of l -fucose exhibit a dwarfed growth habit and decreased wall strength [Reiter, W. D., Chapple, C. & Somerville, C. R. (1993) Science 261, 1032–1035]. Because the mur1 mutation affects several cell wall polysaccharides, whereas the mur2 mutation is specific to xyloglucan, the phenotypes of mur1 plants appear to be caused by structural changes in fucosylated pectic components such as rhamnogalacturonan-II. The normal growth habit and wall strength of mur2 plants casts doubt on hypotheses regarding roles of xyloglucan fucosylation in facilitating xyloglucan–cellulose interactions or in modulating growth regulator activity.

GDP- l -fucose is the activated nucleotide sugar form of l -fucose, which is a constituent of many structural polysaccharides and glycoproteins in various organisms. The de novo synthesis of GDP- l -fucose from GDP- d -mannose encompasses three catalytic steps, a 4,6-dehydration, a 3,5-epimerization, and a 4-reduction. The mur1 mutant of Arabidopsis is deficient in l -fucose in the shoot and is rescued by growth in the presence of exogenously supplied l -fucose. Biochemical assays of the de novo pathway for the synthesis of GDP- l -fucose indicated that mur1 was blocked in the first nucleotide sugar interconversion step, a GDP- d -mannose-4,6-dehydratase. An expressed sequence tag was identified that showed significant sequence similarity to proposed bacterial GDP- d -mannose-4,6-dehydratases and was tightly linked to the mur1 locus. A full-length clone was isolated from a cDNA library, and its coding region was expressed in Escherichia coli . The recombinant protein exhibited GDP- d -mannose-4,6-dehydratase activity in vitro and was able to complement mur1 extracts in vitro to complete the pathway for the synthesis of GDP- l -fucose. All seven mur1 alleles investigated showed single point mutations in the coding region for the 4,6-dehydratase, confirming that it represents the MUR1 gene.

When incubated in the presence of CO gas, Rubrivivax gelatinosus CBS induces a CO oxidation-H ₂ production pathway according to the stoichiometry CO + H ₂ O → CO ₂ + H ₂ . Once induced, this pathway proceeds equally well in both light and darkness. When light is not present, CO can serve as the sole carbon source, supporting cell growth anaerobically with a cell doubling time of nearly 2 days. This observation suggests that the CO oxidation reaction yields energy. Indeed, new ATP synthesis was detected in darkness following CO additions to the gas phase of the culture, in contrast to the case for a control that received an inert gas such as argon. When the CO-to-H ₂ activity was determined in the presence of the electron transport uncoupler carbonyl-cyanide m -chlorophenylhydrazone (CCCP), the rate of H ₂ production from CO oxidation was enhanced nearly 40% compared to that of the control. Upon the addition of the ATP synthase inhibitor N , N ′-dicyclohexylcarbodiimide (DCCD), we observed an inhibition of H ₂ production from CO oxidation which could be reversed upon the addition of CCCP. Collectively, these data strongly suggest that the CO-to-H ₂ reaction yields ATP driven by a transmembrane proton gradient, but the detailed mechanism of this reaction is not yet known. These findings encourage additional research aimed at long-term H ₂ production from gas streams containing CO.

L-Fucose (l-Fuc) is a monosaccharide constituent of plant cell wall polysaccharides and glycoproteins. The committing step in the de novo synthesis of l-Fuc is catalyzed by GDP-d-mannose 4,6-dehydratase, which, in Arabidopsis, is encoded by the GMD1 and GMD2 (MUR1) genes. To determine the functional significance of this genetic redundancy, the expression patterns of both genes were investigated via promoter-β-glucuronidase fusions and immunolocalization of a Fuc-containing epitope. GMD2 is expressed in most cell types of the root, with the notable exception of the root tip where strong expression of GMD1 is observed. Within shoot organs, GMD1::GUS expression is confined to stipules and pollen grains leading to fucosylation of the walls of these cell types in the mur1 mutant. These results suggest that GMD2 represents the major housekeeping gene for the de novo synthesis of GDP-l-Fuc, whereas GMD1 expression is limited to a number of specialized cell types. We conclude that the synthesis of GDP-l-Fuc is controlled in a cell-autonomous manner by differential expression of two isoforms of the same enzyme.