In a study' of the lipopolysaccharides of Pseudomonas syringae and related species, the structure of the O-antigen of P. syringae pv. tabaci strain 223 belonging to serogroup VII has been established. We report now the structure of the O-specific polysaccharide of another representative of this pathovar, namely, P. syringue pv. tabaci strain 225, which belongs to serogroup VIII. As for the loosely bound lipopolysaccharides of many other phytopathogenic pseudomonads'-4, that of strain 225 was isolated from bacteria1 cells by extraction with saline'. The lipopolysaccharide was cleaved with dilute acetic acid and the O-specific polysaccharide was isolated by gel-permeation chromatography on Sephadex G-50. The i3C-n.m.r. spectrum of the polysaccharide (Fig. 1) contained signals for four anomeric carbons at 97.0,97.2, 101.8, and 102.0 p.p.m., three methyl groups (C-6) of 6-deoxy sugars at 17.8-18.1 p.p.m., one hydroxymethyl methyl group (C-6) at 63.0 p.p.m., one carbon bearing nitrogen at 57.2 p.p_m_, 15 other sugar ring carbons in the region 67.8-84.0 p.p_m., and one N-acetyl group (CH, at 23.3 p.p.m., CO at 175.5 p.p.m.).
G.1.c. of alditol acetates derived from the polysaccharide revealed rhamnose and 2-amino-2-deoxyglucose in the ratio -3: 1. Determination6 of the absolute configuration of rhamnose revealed the D and L forms in the ratio -1:2. 2-Amino-Z-deoxyglucase, isolated from the hydrolysate of the polysaccharide by preparative .p.c., had a positive [IX], which indicated the D configuration.
These data indicated that the polysaccharide had a tetrasaccharide repeating-unit that contained residues of D-rhamnose, L-rhamnose (2), and 2-acetamido-2-deoxy-Dglucose.