Structure of repeating unit of the capsular polysaccharide from Acinetobacter baumannii D78 and assignment of the K4 gene cluster

•Capsule structure from Acinetobacter baumannii clinical isolate D78 is disclosed.•The repeating units is branched and consists of four monosaccharide residues.•CPS repeat contains the rare motif (R)-Pyr-(2→4:2→6)-α-d-GalNAc.•Biosynthetic genes were consistent with the structure of the repeating unit.

The structure of the K4 capsular polysaccharide (CPS) from a multiply antibiotic resistant Acinetobacter baumannii isolate D78 was elucidated by chemical and spectroscopical analysis. The K4 repeating unit is composed entirely by aminosugars and the trisaccharide repeating unit of the backbone polysaccharide is branched with a terminal N-acetyl-galactosamine capped with pyruvate as cyclic acetal. Each residue is in the pyranose form, α configured at the anomeric center, and has the D absolute configuration. The genes responsible for the synthesis of the K4 repeat unit and its polymerization and export are found in the capsule locus KL4 present in D78. The pyruvate acetal addition to galactosamine is formed by Ptr1, a novel pyruvate transferase, encoded at this locus.

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