Molecular and functional analysis of genes required for expression of group IB K antigens in Escherichia coli: Characterization of the his-region containing gene clusters for multiple cell-surface polysaccharides

Escherichia coli group I capsular K antigens are found in two forms on the cell surface. The K-LPS form is linked to lipopolysaccharide lipid A core, whereas the high-molecular-weight capsular form is assembled independently of lipid A core. Subgroup IB K antigens are generally co-expressed with either the O8 or O9 antigen and, under the appropriate conditions, with the exopolysaccharide, colanic acid. To examine the relationships between the genetic loci and the synthetic pathways for these various cell-surface polymers, the gene cluster responsible for expression of a prototype group IB K antigen (serotype K40) was cloned and the flanking chromosomal regions characterized. Analysis of the six orfs within the cluster indicates features typical of Wzy (Rfc)-dependent O antigens. Synthesis of group IB K antigens is initiated by WecA (Rfe), a UDP-GlcNAc::undecaprenylphosphate GlcNAc-1-phosphate transferase, and the chain length of K40(LPS) is determined by the wzz gene product. The his-region of the E. coli O8:K40 prototype is almost exclusively devoted to the expression of three different surface polysaccharides. The rfb(K40) cluster is located adjacent to the cps (colanic acid synthesis) and rfb(O8) (O8 antigen synthesis) loci in the gene order: his-rfb(O8/O9)-wzz-ugd-gnd-rfb(K40)-galF-cps. Thus, rfb(K40) is in the location occupied by other Wzy-dependent rfb gene dusters, and rfb(O8/O9) represents an additional locus.