The K90 capsular polysaccharide produced by Acinetobacter baumannii LUH5553 contains di-N-acetylpseudaminic acid and is structurally related to the K7 polysaccharide from A. baumannii LUH5533

Acinetobacter baumannii isolate LUH5553 carries the KL90 capsule gene cluster, which includes genes for three glycosyltransferases (Gtrs) and the ItrA3 initiating transferase, as well as a set of genes for synthesis of a higher sugar, 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-manno-non-2-ulosonic (di-N-acetylpseudaminic) acid (Pse5Ac7Ac). The K90 capsular polysaccharide (CPS) has a tetrasaccharide repeat (K90 unit), which begins with D-GlcpNAc and contains Pse5Ac7Ac. The higher sugar was cleaved by mild acid hydrolysis of the CPS, and structures of the initial and modified polysaccharides were established by 1D and 2D H-1 and C-13 NMR spectroscopy. K90 contains alpha-D-Galp-(1 -> 6)-D-GlcpNAc and alpha-D-GlcpNAc-(1 -> 3)-D-GlcpNAc fragments, and formation of these glycosidic linkages is catalysed respectively by Gtr14 and Gtr15. The gtr14 and gtr15 genes occur in several A. baumannii KL gene clusters, including KL5 and KL7 that carry itrA2 rather than itrA3. As ItrA2 introduces D-GalpNAc rather than D-GlcpNAc as the first monosaccharide, Gtrl 5 can transfer D-GlcpNAc to either of these amino sugars, suggesting that this enzyme has relaxed specificity. Consequently, the third, novel glycosyltransferase, Gtr163, forms the beta-(2 -> 3) linkage between Pse5Ac7Ac and D-Galp. Wzy polymerases encoded by KL90 and KL7 are 54% identical and form the same linkage between the K units to give branched polysaccharides with the same main chain but different disaccharide side chains, beta-Pse5Ac7Ac-(2 -> 3)-D-Galp in K90 and alpha-Leg5Ac7Ac-(2 -> 6)-D-Galp in K7.