One of the most common pathways for the export of O-specific lipopolysaccharide (LPS) across the plasma membrane requires the participation of the Wzx protein. Wzx belongs to a family of integral membrane proteins that share little conservation in their primary amino acid sequence, making it difficult to delineate functional domains. This paper reports the cloning and expression in K-12 of various Wzx homologues from different bacteria as FLAG epitope-tagged protein fusions. A reconstitution system for O16 LPS synthesis was used to assess the ability of each Wzx protein to complement an K-12 Δ mutant. The results demonstrate that Wzx proteins from O-antigen systems that use -acetylglucosamine or -acetylgalactosamine for the initiation of the biosynthesis of the O repeat can fully complement the formation of O16 LPS. Partial complementation was seen with Wzx from , a system that uses -acetylfucosamine in the initiation reaction. In contrast, there was negligible complementation with the Wzx protein from , a system in which galactose is the initiating sugar. These results support a model whereby the first sugar of the O repeat can be recognized by the O-antigen translocation machinery.


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