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 lipopolysa ccharide (LPS) precursor ADP-L-glycero-D-mannoheptose, associated with production of the core oligosaccharide. We have identified an rfaD ho mologue in Vibrio cholerae 01. This gene maps adjacent to the rfb regi on encoding O-antigen biosynthesis, but is transcribed divergently. Th e complete nucleotide sequence of rfaD and the flanking DNA has been d etermined, and rfaD would appear to be the only gene homologous to kno wn LPS core biosynthesis genes in this region. Comparison with the E. coli rfaD shows many similar structural features such as the ADP-bindi ng 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 hom ology, rfaD of V. cholerae may be transcribed using both sigma(70)- an d sigma(54)-dependent promoters.