A PUTATIVE PATHWAY FOR PEROSAMINE BIOSYNTHESIS IS THE FIRST FUNCTION ENCODED WITHIN THE RFB REGION OF VIBRIO-CHOLERAE O1

The first four genes (rfbA,B,D,E) of the rfb region of Vibrio cholerae O1 are predicted to encode the enzymes required for the biosynthesis of perosamine, which constitutes the backbone structure of the O-antig en of the lipopolysaccharide. Based on homology to known proteins/prot ein families, the following functions are predicted: RfbA, phosphomann ose isomerase-guanosine diphosphomannose pyrophosphorylase; RfbB, phos phomanno-mutase; RfbD, oxide reductase and RfbE, perosamine synthetase (amino-transferase). Thus, perosamine is synthesized from fructose 6- phosphate via the intermediates mannose 6-phosphate by RfbA, to mannos e 1-phosphate by RfbB, to GDP-mannose by RfbA, to GDP-4-keto-6-dideoxy mannose by RfbD and to GDP-perosamine by RfbE. This final product woul d then serve as the substrate for the addition of the tetronate, which could then be polymerized into the O-antigen for transfer to the lipi d A plus core oligosaccharide and export to the cell surface. The orga nization of these genes are such that one would expect them to be tran slationally coupled as part of the rfb operon. However, the absence of readily detectable promoter sequences suggests low levels of transcri ption, in line with other studies, The nucleotide sequence of these ge nesis absolutely conserved in the two isolates 569B (classical, Inaba) and O17 (El Tor, Ogawa) which were expected to show maximal sequence variation. This suggests very tight constraints on the micro-evolution within these sequences.