MOLECULAR ARCHITECTURE OF A GALACTOGLUCAN FROM RHIZOBIUM-MELILOTI

Rhizobium meliloti mutants produce a linear, acidic exopolysaccharide with alternating galactopyranosyl and glucopyranosyl units having (1 - -> 3) linkages. It has a 4,6-O-pyruvic cyclic acetal group on the alph a-galactosyl and 6-O-acetyl ester group on the beta-glucosyl units. X- ray diffraction patterns from polycrystalline and well oriented specim ens of its potassium salt indicate that the polymer forms a 2-fold hel ix of pitch 15.89 Angstrom. The three-dimensional structure has been d etermined and refined by using the X-ray intensities and the linked-at om least-squares technique. The details of the antiparallel packing ar rangement of two helices in an orthorhombic unit cell, a = 14.49, b = 9.79, and c = 15.89 Angstrom, reveal that each disaccharide repeating unit is associated with one potassium and three water molecules. The h elices are interconnected by a series of ...COO- ...K-+ ... W... COO- ... interactions. Both pyruvyl and acetyl groups, which are on the per iphery of the helix, are invoked in the association of the polysacchar ide chains and thus appear to be an integral component of the galactog lucan in the nodule invasion process.