The polysaccharide capsules of Escherichia coli have been classified into three groups: I, II, and I/II. The third group, I/II, has been poorly studied and possesses characteristics of both group I and group II capsules. In this report, we describe the cloning of the K10 and K54 capsule gene clusters, two representatives of group I/II capsules. Probes taken from DNA flanking regions 1 and 3 of the group II capsule clusters hybridized to these group I/II clones, confirming that the group I/II capsule genes are flanked by the same DNA and are therefore located in the same serA-linked region of the chromosome as group II capsule gene clusters. Southern blotting showed that homologous sequences were present in both the K10 and K54 capsule gene clusters and in other group I/II strains. No homology was detected between these sequences and the chromosomal DNA of either a group I or a group II strain. Likewise, no homology was detected to the chromosomal DNA of either a K11 or K19 strain, both of which had previously been classified as group I/II strains. In the K10 and K54 capsule gene clusters, these conserved sequences flanked a serotype-specific region in a manner analogous to group II capsule gene organization. Complementation of mutations in the kpsE, kpsD, and kpsC genes in region 1 of the K5 capsule gene cluster by subclones of the K10 and K54 capsule gene clusters indicated that certain stages in the export of group II and I/II capsules may be conserved. In the light of the findings presented here, we suggest that the group I/II capsule gene clusters are sufficiently different from group II capsule gene clusters to justify their renaming as group III.

A gene encoding a positive activator of the expression of extracellular polysaccharide (EPS) synthesis in the phytopathogen Erwinia amylouora has been isolated from a genomic library in Escherichia coli. The presence of the cloned gene in E. culi stimulated transcription of the genes encoding colanic acid biosynthesis and could complement rcsA mutations. Introduction of the gene on a multicopy plasmid into Er. amylouora caused a threefold increase in EPS expression. The nucleotide sequence of the gene (designated VCSA) was determined. This revealed a single open reading frame encoding an RcsA protein of 23.7 kDa. This was confirmed by minicell analysis in E. coli.The predicted amino acid sequence of this RcsA protein showed a high degree of homology to the RcsA protein of Klebsiella aerogenes, demonstrating the existence of a family of related RcsA activator proteins capable of stimulating EPS expression. The protein had no significant homology to known DNA-binding activator proteins, indicating, for the first time, that the RcsA family of activator proteins may stimulate expression of EPS synthesis indirectly by acting on other regulatory proteins.

Analysis of the Escherichia coli K10 capsule gene cluster identified two regions, regions 1 and 3, conserved between different group III capsule gene clusters. Region 1 encodes homologues of KpsD, KpsM, KpsT, and KpsE proteins, and region 3 encodes homologues of the KpsC and KpsS proteins. An rfaH mutation abolished K10 capsule production, suggesting that expression of the K10 capsule was regulated by RfaH in a manner analogous to group II capsule gene clusters. An IS3 element and a φR73-like prophage, both of which may have played a role in the acquisition of group III capsule gene clusters, were detected flanking the K10 capsule genes.

Individual isolates of Escherichia coli synthesize one of more than 70 chemically distinct polysaccharides which form the capsule. In this article we review the genetics of capsule production in E. coli and highlight what this is beginning to reveal in terms of the genetic basis of the structural diversity of polysaccharides. The serA-linked capsule locus can take three different allelic forms. Two of these are associated with capsule genes and are themselves internally variant, whilst the third form has not so far been implicated in capsule biogenesis. Thus the serA-linked region of the E. coli genome is strikingly polymorphic.

Individual isolates of Escherichia coli synthesize one of more than 70 chemically distinct polysaccharides which form the capsule. In this article we review the genetics of capsule production in E. coli and highlight what this is beginning to reveal in terms of the genetic basis of the structural diversity of polysaccharides. The serA‐linked capsule locus can take three different allelic forms. Two of these are associated with capsule genes and are themselves internally variant, whilst the third form has not so far been implicated in capsule biogenesis. Thus the serA‐linked region of the E. coli genome is strikingly polymorphic.