Identification, Genomic Organization, and Analysis of the Group III Capsular Polysaccharide Genes kpsD , kpsM , kpsT , and kpsE from an Extraintestinal Isolate of Escherichia coli (CP9, O4/K54/H5)

Group III capsular polysaccharides (e.g., K54) of extraintestinal isolates ofEscherichia coli , similar to group II capsules (e.g., K1), are important virulence traits that confer resistance to selected host defense components in vitro and potentiate systemic infection in vivo. The genomic organization of group II capsule gene clusters has been established as a serotype-specific region 2 flanked by regions 1 and 3, which contain transport genes that are highly homologous between serotypes. In contrast, the organization of group III capsule gene clusters is not well understood. However, they are defined in part by an absence of genes with significant nucleotide homology to group II capsule transport genes in regions 1 and 3. Evaluation of isogenic, TnphoA -generated, group III capsule-minus derivatives of a clinical blood isolate (CP9, O4/K54/H5) has led to the identification of homologs of the group II capsule transport geneskpsDMTE . These genes and their surrounding regions were sequenced and analyzed. The genomic organization of these genes is distinctly different from that of their group II counterparts. Althoughkps K54 DMTE are significantly divergent from their group II homologs at both the DNA and protein levelsphoA fusions and computer-assisted analyses suggest that their structures and functions are similar. The putative proteins KpsK54 M and KpsK54 T appear to be the integral membrane component and the peripheral ATP-binding component of the ABC-2 transporter family, respectively. The putative KpsK54 E possesses features similar to those of the membrane fusion protein family that facilitates the passage of large molecules across the periplasm. At one boundary of the capsule gene cluster, a truncatedkpsM (kpsM truncated ) and its 5′ noncoding regulatory sequence were identified. In contrast to the completekps K54 M , this region was highly homologous to the group IIkpsM . Fifty-three base pairs 3′ from the end ofkpsM truncated was a sequence 75% homologous to the 39-bp inverted repeat in the IS110 insertion element fromStreptomyces coelicolor . Southern analysis established that two copies of this element are present in CP9. These findings are consistent with the hypothesis that CP9 previously possessed group II capsule genes and acquired group III capsule genes via IS110 -mediated horizontal transfer.