Figure 2. Genetic organization of the O-antigen biosynthesis gene cluster containing wzm-wzt-wbhA.

(A) The genetic locus was identified from the draft genome sequence of the NB10 V. anguillarum strain (unpublished data, E. Hjerde, D.L. Milton, and N.P. Willassen). Each open reading frame (Orf) is indicated by a box and the direction of transcription is indicated by an arrowhead within each box. Grey boxes indicate the genes under investigation in this study, which are suggested to be involved in the transport of O-antigen polysaccharide. wbhA encodes a putative S-adenosylmethionine methyltransferase and wzt and wzm encode components of an ABC-2 subfamily of polysaccharide transporters. The four rml genes have homology to proteins involved in the biosynthesis of dTDP-L-rhamnose, a possible O-antigen precursor [42]. rmlC encodes a dTDP-6-deoxy-D-glucose-3,5-epimerase; rmlD encodes dTDP-6-deoxy-L-mannose dehydrogenase; rmlA encodes glucose-1-phosphate thymidylyltransferase; rmlB encodes dTDP-glucose 4,6-dehydratase. OrfA and OrfB are both part of two putative operons that encode proteins likely involved LPS biosynthesis. Orfs downstream of wbhA encode two putative transposases and a hypothetical protein. (B) A cartoon image of the structure of Gram-negative lipopolysaccharide. The lipopolysaccharide is a component of the outer membrane in Gram-negative bacteria and is composed of lipid A, core polysaccharide, and the O-antigen, which is a repeating sugar unit. OM, outer cell membrane; P, peptidoglycan; IM, inner membrane.