Structure of the K87 capsular polysaccharide and KL87 gene cluster of Acinetobacter baumannii LUH5547 reveals a heptasaccharide repeating unit

K87 capsular polysaccharide (CPS) was isolated from Acinetobacter baumannii isolate LUH5547 that carries the KL87 capsule biosynthesis gene cluster at the chromosomal K locus. Studies by sugar analysis, selective chemical cleavages, and 1D and 2D ¹H and ¹³C NMR spectroscopy showed that the CPS has a branched heptasaccharide repeat (K unit) containing one residue each of D-glucose (D-Glсp), D-glucuronic acid (D-GlсpA), N-acetyl-D-glucosamine (D-GlсpNAc), 6-deoxy-L-talose (L-6dTalp), and three residues of L-rhamnose (L-Rhap). The following structure of the CPS was established:

→3)−α-L-Rhap−(1→2)-α-L-Rhap-(1→3)-α-L-6dTalp-(1→3)-β-D-GlcpNAc-(1→

        2

        ↑

        1

β-D-GlcpA-(4←1)-α-D-Glcp(2←1)-α-L-Rhap

The position of a minor O-acetyl group present in the CPS was not determined. Functions of enzymes coded by genes in the KL87 gene cluster were tentatively assigned and found to be consistent with the CPS structure.