BIOSYNTHESIS OF LIPID-LINKED OLIGOSACCHARIDES IN RHIZOBIUM-LEGUMINOSARUM

As a first step in the study of the biosynthesis of Rhizobium legumino sarum 8401 exopolysaccharides (EPS) the formation of lipid-linked deri vatives that could act as intermediate carriers has been investigated. UDP-GlcA and UDP-Glc, either [C-14]labeled or not, were used as sugar precursors and EDTA-treated calls as enzyme system. This preparation contains also endogenous donors and lipidic acceptors. Seven lipid-lin ked sugars were isolated and partially characterized in chloroform/met hanol/water (1:2:0.3) extracts, as judged by paper chromatography and electrophoresis, DEAE-cellulose ion exchange chromatography and gel fi ltration. The structure proposed for the repeat unit of the EPS under study is(Lamb et al., 1982, Mol. Gen. Genet. 186, 449-452): [GRAPHICS] The results obtained suggest that the octasaccharide repeat unit (com pound A) with one (compound B) or two (compound C) ketal pyruvate resi dues are assembled on a lipid acceptor. In addition, a lipid-linked tr i-saccharide GlcA-->(beta)4GlcA-->(beta)4Glc (compound D-1) and its ac etylated derivative (compoundD(2)) were also detected. A sixth compoun d, containing only glucose-linked lipid, could be the first step in. t he assembly of the repeat unit. All these compounds are assumed to be intermediates in the biosynthesis of the 8401 EPS. The seventh lipid-l inked compound contained only GlcA. These studies anal characterizatio ns have been performed in comparison. to a Rhizobium leguminosarum bv. trifolii NA-30 system that produces similar lipid-linked derivatives, anal an EPS with the same structure (J. C. Bossio et al., 1986, Bioch em. Biophys. Res. Commun. 134, 205-211).