title: Roles of predicted glycosyltransferases in the biosynthesis of the Rhizobium etli CE3 O antigen.
abstract: The Rhizobium etli CE3 O antigen is a fixed-length heteropolymer. The genetic regions required for its synthesis have been identified, and the nucleotide sequences are known. The structure of the O antigen has been determined, but the roles of specific genes in synthesizing this structure are relatively unclear. Within the known O-antigen genetic clusters of this strain, nine open reading frames (ORFs) were found to contain a conserved glycosyltransferase domain. Each ORF was mutated, and the resulting mutant lipopolysaccharide (LPS) was analyzed. Tricine SDS-PAGE revealed stepwise truncations of the O antigen that were consistent with differences in mutant LPS sugar compositions and reactivity with O-antigen-specific monoclonal antibodies. Based on these results and current theories of O-antigen synthesis, specific roles were deduced for each of the nine glycosyltransferases, and a model for biosynthesis of the R. etli CE3 O antigen was proposed. In this model, O-antigen biosynthesis is initiated with the addition of N-acetyl-quinovosamine-phosphate (QuiNAc-P) to bactoprenol-phosphate by glycosyltransferase WreU. Glycosyltransferases WreG, WreE, WreS, and WreT would each act once to attach mannose, fucose, a second fucose, and 3-O-methyl-6-deoxytalose (3OMe6dTal), respectively. WreH would then catalyze the addition of methyl glucuronate (MeGlcA) to complete the first instance of the O-antigen repeat unit. Four subsequent repeats of this unit composed of fucose, 3OMe6dTal, and MeGlcA would be assembled by a cycle of reactions catalyzed by two additional glycosyltransferases, WreM and WreL, along with WreH. Finally, the O antigen would be capped by attachment of di- or tri-O-methylated fucose as catalyzed by glycosyltransferase WreB.
authors: Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin, USA.
authors: Kristylea J
authors: KJ
authors: Ojeda
authors: Laurie
authors: L
authors: Simonds
authors: K Dale
authors: KD
authors: Noel
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D005643:Fucose / Q000378:metabolism; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism*; D015394:Molecular Structure; D019081:O Antigens / Q000096:biosynthesis* / Q000737:chemistry; D016366:Open Reading Frames; D042163:Rhizobium etli / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D019081:O Antigens; D005643:Fucose; D016695:Glycosyltransferases