In this pathway, O-antigens are completely synthesized on the cytosolic leaflet of the plasma membrane. Undecaprenyl-phosphate (black line and yellow circle) serves as the lipid acceptor and is modified by the addition of an acetylated amino sugar phosphate (frequently N-acetylglucosamine-1-P, white hexagon) as well as two or more additional sugar residues (grey hexagons) to generate a biosynthesis primer. The polymerizing enzyme(s) extend the primer with tens to hundreds of O-antigen repeat units (light blue hexagons). In some species, termination of O-antigen biosynthesis is achieved by modifying the polymer’s non-reducing end (black star). An ABC transporter translocates the UND-PP-linked O-antigen intermediate to the membrane’s periplasmic side, where it forms a substrate for glycosylation of the lipopolysaccharide (LPS) core. Only transporters translocating terminally modified O-antigens contain CBDs that bind the polysaccharide’s modified terminus.