The structures of the lipopolysaccharides from Rhizobium etli strains CE358 and CE359. The complete structure of the core region of R. etli lipopolysaccharides.

The structural arrangement of oligosaccharides comprising the core region of Rhizobium etli CE3 lipopolysaccharide (LPS) has been elucidated through the characterization of the LPSs from two R. etli mutants. One mutant, CE358, completely lacks the O-chain polysaccharide, while the second mutant, CE359, contains a truncated portion of this polysaccharide. This structural arrangement of the core oligosaccharides in these LPSs was determined using electrospray ionization mass spectrometry, tandem mass spectrometry, and methylation analysis. Mild acid hydrolysis of the CE359 LPS produces two major core oligosaccharides: a tetrasaccharide (1) with the structure alpha-D-Galp-(1-->6)-[alpha-D-GalpA-(1-->4)]-alpha-D-Manp-(1 -->5)-Kdo p (where Kdo represents 3-deoxy-D-manno-2-octulosonic acid) and a trisaccharide (2) having the structure alpha-D-GalpA-(1-->4)-[alpha-D-GalpA-(1-->5)]-Kdop. Structure 1 in CE358 LPS lacks the galacturonosyl residue. Glycosyl linkage and tandem mass spectrometry analyses show that the intact LPS core region consists of trisaccharide (2) attached to O-4 of the Kdo residue in tetrasaccharide 1, and that an additional Kdo residue is attached to O-6 of the galactosyl residue of 1. [structure: see text] The additional terminally linked Kdo residue is not in close proximity to the lipid A moiety, a unique location for a core Kdo residue. The mutant LPS preparations also contain minor LPS species, one of which lacks the Kdo linked to O-6 of the galactosyl residue, another that lacks the galacturonic acid attached to O-5 of Kdo, and a third that lacks two galacturonosyl residues and one Kdo residue. Thus, in addition to lacking both heptose and phosphate, the R. etli LPS core region differs substantially from the typical enterobacterial cores. The abundance of galacturonosyl residues in the R. etli core might serve as a suitable functional replacement for phosphate, such as would be predicted for Ca2+ binding.