Two extremely divergent sequence forms of the genes that define Escherichia coli group 3 capsules suggest a very long history since their common ancestor.
Capsules are a critical virulence factor in many pathogenic Escherichia coli, of which groups 2 and 3 capsules are synthesised by the ABC transporter pathway. The well-studied forms are in group 2 and much of our knowledge of group 3 is inferred from our understanding of group 2. We analyse six group 3 gene clusters including representatives of K10, K11 and K96, and find unexpected diversity. Groups 2 and 3 both have gene clusters with terminal regions 1 and 3 containing mostly genes shared by all members of both groups, plus a central region 2, that in group 2 has the genes for synthesising the serotype-specific repeat unit. We find that in all but one case group 3 gene clusters include, in addition to serotype-specific genes, a previously unrecognised set of shared genes in region 2 that probably codes for an additional structural element. Also, the six shared genes in regions 1 and 3 of group 3 exist in two very different sequence forms. It appears that the E. coli ABC transporter capsules have a very long history, with more fundamental diversity present in group 3, but greater diversity in the exposed strongly antigenic serotype-specific component encoded by region 2.