Analysis of the first two genes of the CS1 fimbrial operon in human enterotoxigenic Escherichia coli of serotype 0139:H28

Evolutionary and functional relationships of colonization factor antigen i and other class 5 adhesive fimbriae of enterotoxigenic Escherichia coli

Colonization factor antigen I (CFA/I) is the archetype of eight genetically related fimbriae of enterotoxigenic Escherichia coli (ETEC) designated class 5 fimbriae. Assembled by the alternate chaperone pathway, these organelles comprise a rigid stalk of polymerized major subunits and an apparently tip-localized minor adhesive subunit. We examined the evolutionary relationships of class 5-specific structural proteins and correlated these with functional properties. We sequenced the gene clusters encoding coli surface antigen 4 (CS4), CS14, CS17, CS19, and putative colonization factor antigen O71 (PCFO71) and analyzed the deduced proteins and the published homologs of CFA/I, CS1, and CS2. Multiple alignment and phylogenetic analysis of the proteins encoded by each operon define three subclasses, 5a (CFA/I, CS4, and CS14), 5b (CS1, CS17, CS19, and PCFO71), and 5c (CS2). These share distant evolutionary relatedness to fimbrial systems of three other genera. Subclass divisions generally correlate with distinguishing in vitro adherence phenotypes of strains bearing the ETEC fimbriae. Phylogenetic comparisons of the individual structural proteins demonstrated greater intrasubclass conservation among the minor subunits than the major subunits. To correlate this with functional attributes, we made antibodies against CFA/I and CS17 whole fimbriae and maltose-binding protein fusions with the amino-terminal half of the corresponding minor subunits. Anti-minor subunit Fab preparations showed hemagglutination inhibition (HAI) of ETEC expressing homologous and intrasubclass heterologous colonization factors while anti-fimbrial Fab fractions showed HAI activity limited to colonization factor-homologous ETEC. These results were corroborated with similar results from the Caco-2 cell adherence assay. Our findings suggest that the minor subunits of class 5 fimbriae may be superior to whole fimbriae in inducing antiadhesive immunity.

Use of a novel approach, termed island probing, identifies the Shigella flexneri she pathogenicity island which encodes a homolog of the immunoglobulin A protease-like family of proteins

The she gene of Shigella flexneri 2a, which also harbors the internal enterotoxin genes set1A and set1B (F. R. Noriega, GenBank accession no. U35656, 1995) encodes a homolog of the virulence-related immunoglobulin A (IgA) protease-like family of secreted proteins, Tsh, EspC, SepA, and Hap, from an avian pathogenic Escherichia coli, an enteropathogenic E. coli, S. flexneri 5, and Haemophilus influenzae, respectively. To investigate the possibility that this locus was carried on a larger deletable element, the S. flexneri 2a YSH6000T she gene was insertionally disrupted by allelic exchange using a Tn10-derived tetAR(B) cassette. Then, to detect loss of the she locus, the tetracycline-resistant derivative was plated onto fusaric acid medium to select for tetracycline-sensitive revertants, which were observed to arise at a frequency of 10(-5) to 10(-6). PCR and pulsed-field gel electrophoresis analysis confirmed loss of the she::tetAR(B) locus in six independent tetracycline-sensitive isolates. Sample sequencing over a 25-kb region flanking she identified four insertion sequence-like elements, the group II intron-like sequence Sf.IntA, and the 3' end of a second IgA protease-like homolog, sigA, lying 3.6 kb downstream and in an orientation inverted with respect to she. The deletion was mapped to chromosomal NotI fragment A and determined to have a size of 51 kb. Hybridization with flanking probes confirmed that at least 17.7 kb of the 51-kb deletable element was unique to the seven she+ strains investigated, supporting the conclusion that she lay within a large pathogenicity island. The method described in this study, termed island probing, provides a useful tool to further the study of pathogenicity islands in general. Importantly, this approach could also be of value in constructing safer live attenuated bacterial vaccines.

Transcriptional control of genes encoding CS1 pili: negative regulation by a silencer and positive regulation by Rns

The adherence of enterotoxigenic Escherichia coli (ETEC) to the human small intestine is an important early event in infection. Attachment is thought to be mediated by proteinaceous structures called pili. We have investigated the regulation of expression of the genes encoding CS1 pili found on human ETEC strains and find that there are at least three promoters, P1 and P2, upstream of the coo genes, and P3, downstream of the start of cooB translation. We identified a silencer of transcription which extends over several hundred bases overlapping the cooB open reading frame. This silencer is dependent on the promoter and/or upstream region for its negative effect. The DNA binding protein H-NS is a repressor of coo transcription that acts in the same region as the silencer, so it is possible that H-NS is involved in this silencing. Rns, a member of the AraC family, positively regulates transcription of the coo operon and relieves the silencing of CS1 expression.

CooC and CooD are required for assembly of CS1 pili

Many strains of enterotoxigenic Escherichia coli (ETEC) isolated from patients with diarrhoeal disease exhibit CS1 pili on their surfaces. These appendages, which are thought to be important for colonization of the upper intestine, are composed largely of multiple identical protein subunits encoded by cooA. We have sequenced the DNA directly downstream of cooA and identified two open reading frames, cooC and cooD, transcribed in the same direction as cooB and cooA. Following cooD is DNA homologous to an insertion sequence, so cooB, A, C and D appear to encode all the information needed for E. coli K-12 to synthesize CS1 pili. Complementation analysis of mutants cloned in E. coli K-12 and constructed in an ETEC-derived strain indicates that cooC and cooD are not required for stability of the major CS1 pilin protein or for its transport to the periplasm, but, like cooB, both are needed for assembly of cooA into pili.

Differential decay of RNA of the CFA/I fimbrial operon and control of relative gene expression

CFA/I fimbriae on human enterotoxigenic Escherichia coli are composed of the CfaB protein, the product of the second gene of the CFA/I operon. We show here that CfaB is expressed at a higher level than other proteins of the CFA/I operon. mRNA encoding the CfaB protein is much more abundant than mRNA encoding CfaA, the first protein, together with CfaB or mRNA encoding CfaA only. Only one promoter, upstream of cfaA, is present. These data indicate that a primary transcript containing cfaA and cfaB is processed into a cfaA-specific mRNA and a cfaB-specific mRNA. The cfaA mRNA is unstable, while the cfaB mRNA is stable and therefore accumulates in CFA/I-producing E. coli. The cfaB mRNA is probably stabilized by a stem-loop structure downstream of the cfaB gene. No distinct mRNA fragments could be detected encoding the other two proteins, CfaC and CfaE, of the CFA/I operon. These results indicate that cfaC- and cfaE-specific mRNAs degrade very rapidly and/or are produced in small amounts.