Identification of an ATP-binding cassette transporter for export of the O-antigen across the inner membrane in Rhizobium etli based on the genetic, functional, and structural analysis of an lps mutant deficient in O-antigen

For O-antigen lipopolysaccharide (LPS) synthesis in bacteria, transmembrane migration of undecaprenyl pyrophosphate-bound O-antigen oligosaccharide su bunits or polysaccharide occurs before ligation to the core region of the L PS molecule. In this study, we identified by mutagenesis an ATP-binding cas sette transporter in Rhizobium etli CE3 that is likely responsible for the translocation of the O-antigen across the inner plasma membrane. Mutant FAJ 1200 LPS lacks largely the O-antigen, as shown by SDS-polyacrylamide gel el ectrophoresis and confirmed by immunoblot analysis. Furthermore, LPS isolat ed from FAJ1200 is totally devoid of any O-chain glycosyl residues and cont ains only those glycosyl residues that can be expected for the inner core r egion. The membrane component and the cytoplasmic ATP-binding component of the ATP-binding cassette transporter are encoded by wzm and wzt, respective ly. The Tn5 transposon in mutant FAJ1200 is inserted in the wzm gene. This mutation resulted in an Inf phenotype in bean plants.