Genetics of O-antigen biosynthesis in Pseudomonas aeruginosa

Pathogenic bacteria produce an elaborate assortment of extracellular and ce ll-associated bacterial products that enable colonization and establishment of infection within a host. Lipopolysaccharide (LPS) molecules are cell su rface factors that are typically known for their protective role against se rum-mediated lysis and their endotoxic properties. The most heterogeneous p ortion of LPS is the O antigen or O polysaccharide, and it is this region w hich confers serum resistance to the organism. Pseudomonas aeruginosa is ca pable of concomitantly synthesizing two types of LPS referred to as A band and B band The A-band LPS contains a conserved O polysaccharide region comp osed of D-rhamnose (homopolymer), while the B-band O-antigen (heteropolymer ) structure varies among the 20 O serotypes of P. aeruginosa. The genes cod ing for the enzymes that direct the synthesis of these two O antigens are o rganized into two separate clusters situated at different chromosomal locat ions. In this review, we summarize the organization of these two gene clust ers to discuss how A-band and B-band O antigens ape synthesized and assembl ed by dedicated enzymes. Examples of unique proteins required for both A-ba nd and B-band O-antigen synthesis and for the synthesis of both LPS and alg inate are discussed. The recent identification of additional genes within t he P. aeruginosa genome that are homologous to those in the A-band and B-ba nd gene clusters are intriguing since some are able to influence O-antigen synthesis. These studies demonstrate that P. aeruginosa represents a unique model system, allowing studies of heteropolymeric and homopolymeric O-anti gen synthesis, as well as permitting an examination of the interrelationshi p of the synthesis of LPS molecules and other virulence determinants.