A putative pathway for biosynthesis of the O-antigen component, 3-deoxy-L-glycero-tetronic acid, based on the sequence of the Vibrio cholerae 01 rfb region

The nucleotide sequence of a region of the rfb genes, encoding biosynthesis of the Vibrio cholerae (Vc) O1O-antigen, was determined. Analysis of the open reading frames (ORFs) within this region has revealed similarities with a number of different classes of biosynthetic proteins and enzymes. The ORFs have been designated RfbK, RfbL, RfbM, RfbN and RfbO. RfbK is a small, acidic protein which has similarity to the family of proteins known as acyl-carrier proteins (ACP). The RfbL protein has similarity to a super-family of enzymes which adenylate their substrates as a part of their reaction mechanism. Included in these are several acetyl-CoA ligases. Alignment of RfbL with these proteins reveals a highly conserved domain containing the motif GlyXaaXaaGlyXaaPro. This resembles the ATP-binding site motif and may represent a variant of the usual motif, except that Pro replaces Gly. The VcRfbM protein has similarity with a family of long-chain, iron-containing alcohol dehydrogenases, of which the Escherichia coli K-12 fucO and adhE gene products are also members. The RfbN protein has sequence homology with LuxE and LuxC of Vibrio harveyi (Vh) and other bioluminescent bacterial species. The latter are two components of the enzyme complex which synthesizes the long-chain aldehyde used in the V. harveyi bioluminescence system. Finally, the VcRfbO protein has sequence similarity with acetyl-CoA transferases. We were able to identify a number of the gene products using a T7 expression system, confirming several of the allocated ORFs. A biosynthetic pathway for the Vc O-antigen component 3-deoxy-L-glycero-tetronic acid, based on the enzymatic functions predicted for the RfbK, RfbL, RfbM, RfbN and RfbO proteins, is presented.