Plays a role in the biosynthesis of B-band O antigen for serotype O5. Catalyzes the transfer of an acetyl group to the C-3 amino position of UDP-2-acetamido-3-amino-2,3-dideoxy-alpha-D-glucuronate (UDP-D-GlcNAc3NA). acetyl-CoA + UDP-2-acetamido-3-amino-2,3-dideoxy-alpha-D-glucuronate = CoA + H(+) + UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-glucuronate kcat is 2900 min(-1). Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homotrimer. Cells lacking this gene are incapable of producing either long-chain B-band O antigen (> or = 2 repeating units) or semi-rough LPS (lipid A-core + one O antigen repeat). Belongs to the transferase hexapeptide repeat family.