The K17 capsular polysaccharide (CPS) produced by Acinetobacter baumannii G7, which carries the KL17 configuration at the capsule biosynthesis locus, was isolated and studied by chemical methods along with one- and two-dimensional 1H and 13C NMR spectroscopy. Selective cleavage of the glycosidic linkage of a 2,4-diacetamido-2,4,6-trideoxy-D-glucose (D-QuiNAc4NAc) residue by (i) trifluoroacetic acid solvolysis or (ii) alkaline β-elimination (NaOH-NaBH4) of the 4-linked D-alanine amide of a 2-acetamido-2-deoxy-D-galacturonic acid residue (D-GalNAcA6DAla) yielded trisaccharides that were isolated by Fractogel TSK HW-40 gel-permeation chromatography and identified by using NMR spectroscopy and high-resolution electrospray ionization mass spectrometry. The following structure was established for the trisaccharide repeat (K unit) of the CPS: →4)-α-D-GalpNAcA6DAla-(1→4)-α-D-GalpNAcA-(1→3)-β-D-QuipNAc4NAc-(1→ . The presence of the itrA1 gene coding for the initial glycosylphosphotransferase in the KL17 gene cluster established the first sugar of the K unit as D-QuipNAc4NAc. KL17 includes genes for three transferases that had been annotated previously as glycosyltransferases (Gtrs). As only two Gtrs are required for the K17 structure and one D-GalpNAcA residue is modified by a D-alanine amide, these assignments were re-assessed. One transferase was found to belong to the ATPgrasp_TupA protein family that includes D-alanine-D-alanine ligases, and thus was renamed Alt1 (alanine transferase). Alt1 represents a novel family that amidate the carboxyl group of D-GalpNAcA or D-GalpA.