The WbaP (formerly RfbP) protein has been characterized as the first enzyme in O-antigen biosynthesis in Salmonella typhimurium. The enzyme transfers galactose from UDP-galactose to a polyprenyl carrier (utilizing the highly conserved C-terminal sugar transferase domain, pfam02397) a reaction which takes place at the cytoplasmic face of the inner membrane. The N-terminal hydrophobic domain is then believed to facilitate the "flippase" function of transferring the liposaccharide unit from the cytoplasmic face to the periplasmic face of the inner membrane. This model includes the enterobacterial enzymes, where the function is presumed to be identical to the S. typhimurium enzyme as well as a somewhat broader group which are likely to catalyze the same or highly similar reactions based on a phylogenetic tree-building analysis of the broader sugar transferase family. Most of these genes are found within large operons dedicated to the production of complex exopolysaccharides such as the enterobacterial O-antigen. The most likely heterogeneity would be in the precise nature of the sugar molecule transferred.
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