SYNTHESIS OF ESCHERICHIA-COLI-O9A POLYSACCHARIDE REQUIRES THE PARTICIPATION OF 2 DOMAINS OF WBDA, A MANNOSYLTRANSFERASE ENCODED WITHIN THE WB-ASTERISK-GENE-CLUSTER

WbdA (previously MtfA) is one of the mannosyltransferases encoded with in the Escherichia coli O9a wb gene cluster. It is composed of two do mains of similar size, connected by an alpha-helix chain. Elimination of the C-terminal half by transposon insertion or gene deletion caused synthesis of an altered structural O-polysaccharide consisting only o f alpha-1,2-linked mannose. O9a polysaccharide synthesis was restored by the C-terminal half of WbdA in trans. No membrane incorporation of mannose from GDP mannose was observed in a strain carrying only the ge ne for truncated WbdA. For mannose incorporation, it was necessary to introduce both wbdB and wbdC genes into the strain, Therefore, it is l ikely that the N-terminal half of truncated WbdA synthesizes the alter ed O-polysaccharide together with other mannosyltransferases which par ticipate in the initial reactions of the O9a polysaccharide synthesis. Both N- and C-terminal domains of WbdA are required for the synthesis of the complete E. coli O9a polysaccharide. The chi sequence location between the two domains and homology plot analyses of the wbdA and th e WbdA protein suggested that the wbdA gene might have arisen by fusio n of two independent genes.