Structural characterization of the symbiotically important low-molecular-weight succinoglycan of Sinorhizobium meliloti

The production of succinoglycan by Sinorhizobium meliloti Rm1021 is require d for successful nodule invasion by the bacterium of its host plant, alfalf a. Rm1021 produces succinoglycan, an acidic exopolysaccharide composed of a n octasaccharide repeating unit modified,vith acetyl, succinyl, and pyruvyl moieties, in both low- and high-molecular-weight forms. Low-molecular-weig ht (LMW) succinoglycan, previously thought to consist of monomers, trimers, and tetramers of the repeating unit, has been reported as being capable of promoting the formation of nitrogen-fixing nodules by succinoglycan-defici ent derivatives of strain Rm1021. We have determined that the three size cl asses of LMW succinoglycan species are in fact monomers, dimers, and trimer s of the repeating unit and that the trimer is the species active in promot ing nodule invasion. A detailed structural analysis of the components of LM W succinoglycan by using various chromatographic techniques, along with nuc lear magnetic resonance analyses, has revealed that there is considerable h eterogeneity within the LMW succinoglycan oligomers in terms of noncarbohyd rate substitutions, and we have determined the structural basis of this het erogeneity.