Undecaprenyl-linked O antigen polysaccharide precursors (O-PS), initiated by a WecA homologue and elongated at the nonreducing end (black dots), are polymerized in the cytosol by the Wbd glycosyltransferases, shown in purple. Undecaprenyl-linked O antigen polysaccharide precursors (O-PS), initiated by a WecA homologue and elongated at the nonreducing end (black dots), are polymerized in the cytosol by the Wbd glycosyltransferases, shown in purple. The ABC transporter formed by Wzm and Wzt is required for transfer of these undecaprenyl-linked polymers to the periplasmic face of the membrane (left side of the figure). The core-KDO-lipid A complex is synthesized by the Lpx proteins, and the addition of sugar residues (blue dots) that constitute the core polysaccharide is catalyzed by the Waa proteins. KDO-lipid A is flipped from the inner face to the outer face of the membrane by the MsbA ABC transporter (right side of the figure). The WaaL ligase, shown in green, ligates O-PS to KDO-lipid A to form LPS. Export of LPS to the outer membrane is dependent on the LptABC proteins (formerly called YhbN, YhbG, and YrbK), forming an ABC transporter with LptFG (formerly called YjgP and YjgQ) (middle part of the figure). Additional proteins (?) might participate in this step. The LptDE proteins (formerly called Imp and Rlp) are required to translocate LPS to the outer face of the outer membrane. This figure is based on data from references 41, 391, 408a, and 522. Amy L. Davidson et al. Microbiol. Mol. Biol. Rev. 2008; doi:10.1128/MMBR.00031-07