IN VIBRIO-CHOLERAE SEROGROUP O1, RFAD IS CLOSELY LINKED TO THE RFB OPERON

The rfaD gene of Escherichia coli encodes ADP-L-glycero-D-mannoheptose-6-epimerase an enzyme required for the biosynthesis of the lipopolysaccharide (LPS) precursor ADP-L-glycero-D-mannoheptose, associated with production of the core oligosaccharide. We have identified an rfaD homologue in Vibrio cholerae 01. This gene maps adjacent to the rfb region encoding O-antigen biosynthesis, but is transcribed divergently. The complete nucleotide sequence of rfaD and the flanking DNA has been determined, and rfaD would appear to be the only gene homologous to known LPS core biosynthesis genes in this region. Comparison with the E. coli rfaD shows many similar structural features such as the ADP-binding beta alpha beta fold at the N terminus, as well as a high degree of homology of both the nucleotide and aminoacid sequences. Based on homology, rfaD of V. cholerae may be transcribed using both sigma(70)- and sigma(54)-dependent promoters.