Biosynthesis of UDP-α--Acetyl-dmannosaminuronic Acid and CMP-β--Acetyl-d-neuraminic Acid for the Capsular Polysaccharides of .

is a human pathogen and a leading cause of food poisoning in North America and Europe. The exterior surface of the bacterial cell wall is attached to a polymeric coat of sugar molecules known as the capsular polysaccharide (CPS) that helps protect the organism from the host immune response. The CPS is composed of a repeating sequence of common and unusual sugar residues. In the HS:11 serotype of , we identified two enzymes in the gene cluster for CPS formation that are utilized for the biosynthesis of UDP-α--acetyl-d-mannosaminuronic acid (UDP-ManNAcA). In the first step, UDP-α--acetyl-d-glucosamine (UDP-GlcNAc) is epimerized at C2 to form UDP-α--acetyl-d-mannosamine (UDP-ManNAc). This product is then oxidized by a NAD-dependent C6-dehydrogenase to form UDP-ManNAcA. In the HS:6 serotype ( strain 81116), we identified three enzymes that are required for the biosynthesis of CMP-β--acetyl-d-neuraminic acid (CMP-Neu5Ac). In the first step, UDP-GlcNAc is epimerized at C2 and subsequently hydrolyzed to form -acetyl-d-mannosamine (ManNAc) with the release of UDP. This product is then condensed with PEP by -acetyl-d-neuraminate synthase to form -acetyl-d-neuraminic acid (Neu5Ac). In the final step, CMP--acetyl-d-neuraminic acid synthase utilizes CTP to convert this product into CMP-Neu5Ac. A bioinformatic analysis of these five enzymes from serotypes HS:11 and HS:6 identified other bacterial species that can produce UDP-ManNAcA or CMP-Neu5Ac for CPS formation.