C-TERMINAL HALF OF SALMONELLA-ENTERICA WBAP (RFBP) IS THE GALACTOSYL-1-PHOSPHATE TRANSFERASE DOMAIN CATALYZING THE FIRST STEP OF O-ANTIGEN SYNTHESIS

We previously showed that tile product of the wbaP gene of Salmonella enterica serovar Typhimurium has two functions: it is involved in the first step of O-antigen synthesis (the galactosyltransferase [GT] func tion) and in a later step (the T function), first thought to be the fl ipping of the O-antigen subunit on undecaprenyl pyrophosphate from the cytoplasmic face to the periplasmic face of the cytoplasmic membrane, We now locate two wbaP(T) mutations within the first half of the wbaP gene by sequencing, Both mutants retain GT activity, although one was a frameshift mutation resulting in a stop codon 10 codons after the f rameshift to give an open reading frame containing only 138 of the 476 codons in WbaP. We also show that there is a secondary translation st arting within the wbaP gene resulting in the synthesis of a polypeptid e with GT activity. These results indicate that the N- and C-terminal halves of WhaP are the T and GT functional domains, respectively, We n ow propose that the T block operates prior to the flippase function, p robably at the release of undecaprenyl pyrophosphate-linked galactose from WbaP.