The complete structure of lipid a from Rhizobium giardinii bv. giardinii CAS 24 the bacterial endophyte of white Spanish broom (Chamaecytisus albus).

The endophyte of Chamaecytisus albus named Rhizobium sp. CAS 24 is a soil bacterium belonging to Rhizobium giardinii bv. giardinii species, as was established by multilocus sequence analysis. Chemical analysis, mass spectrometry and NMR spectroscopy, revealed that the CAS 24 lipid A consist species bearing tri-, tetra- and penta-acyl residues. All share the same glucosaminyl-(1 → 6)-glucosamine backbone which is mono- or di-phosphorylated. The penta-acylated lipid A contain 14:0(3-OH) ester-linked acids with free 3-hydroxyls at the C-3 and C-3' positions of the backbone. Two amide-linked residues (18:0(3-OH) and 18:1(3-OH)) occupy positions C-2 and C-2', respectively. The 18:1(3-OH) is esterified on its hydroxyl by a 29-hydroxytriacontanoic acid. The tetra-acylated lipid A lack the 14:0(3-OH) on proximal GlcpN while the tri-acylated lipid A species lack both 14:0(3-OH). Other variants of lipid A CAS 24 were created by substituting the acyl residues indicated above on the sugar backbone with their chain homologues. Genome sequence analyses of the CAS 24 strain revealed that the genes encoding the lipid A biosynthetic pathway showed significant sequence similarity (close to 100%) and gene organization to the corresponding genes of Sinorhizobium melilotii and S. fredii. The presented structure of the CAS 24 lipid A is similar but not identical to that previously described for Sinorhizobium fredii HH103.