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Biosynthesis of membrane-derived oligosaccharides. Novel glucosyltransferase system from Escherichia coli for the elongation of beta 1----2-linked polyglucose chains.

Membrane-derived oligosaccharides (MDO) of Escherichia coli are a family of substituted branched oligomers containing 8-12 residues of glucose that are joined by beta 1----2 and beta 1----6 linkages. MDO are localized in the periplasmic space of the cell, and their biosynthesis is regulated by the osmolarity of the medium (Kennedy, E. P. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 1092-1095). We report here the initial characterization of a novel glucosyltransferase system that catalyzes the elongation of beta 1----2-linked polyglucose chains. The system requires: 1) a beta-D-glucoside such as the disaccharide sophorose (2-O-beta-D-glucosyl-glucose) or octyl beta-D-glucoside; 2) a trypsin-sensitive membrane fraction; 3) a heat-stable protein from the soluble fraction; 4) UDP-glucose; and 5) Mg2+ ions. Oligomers containing 6-10 glucose units (about the same size as MDO) that are joined by beta 1----2 linkages are major products of the enzyme system. Mutants in the recently mapped mdoA locus (Bohin, J. -P., and Kennedy, E. P. (1984) J. Bacteriol. 157, 956-957) are blocked in vivo at an early stage of MDO synthesis. It has now been found that mdoA mutants are defective in the membrane component, but not in the heat-stable protein that is required for the in vitro synthesis of beta 1----2-linked glucosyl oligomers. We conclude that the glucosyltransferase system described here has an essential function in the synthesis of MDO in vivo.