Molecular analysis of the cso operon of enterotoxigenic Escherichia coli reveals that CsoA is the adhesin of CS1 fimbriae and that the accessory genes are interchangeable with those of the cfa operon

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Abstract
A deletion mutation in csoA, the gene encoding the structural subunit protein of CS1 fimbriae of enterotoxigenic Escherichia coli of serotype O6:K15:H16 or H-, was constructed in the subcloned CS1 genetic determinant. The mutation resulted in the abolition of CS1 fimbrial adhesiveness. Complementation, in trans, involving the determinant with the csoA deletion mutation and the gene encoding the structural subunit protein, CsoA, expressed from compatible plasmids, restored the expression and adhesive ability of CS1 fimbriae. In addition, trans-complementation was achieved between the cso determinant with the aforementioned deletion mutation and the cfaB gene encoding the structural subunit protein (CfaB) of CFA/I fimbriae, resulting in the expression of CFA/I fimbriae. The observation that heterologous assembly was possible between these two fimbrial systems, together with the knowledge that the adhesin of CFA/I fimbriae is the structural subunit, was exploited to investigate whether CsoA had adhering properties. A deletion mutation in cfaB was created in the CFA/I fimbrial determinant. Complementation of this mutation with csoA in trans resulted in expression of the CsoA antigen on the bacterial cell surface and restoration of bacterial adherence. As no minor subunits act as the adhesin in CFA/I fimbriae, adhesion was mediated by CsoA. Nucleotide sequencing of the DNA region downstream from csoA confirmed the absence of genes encoding minor subunits which might act as the adhesin. Two open reading frames were revealed which encoded proteins sharing considerable homology with proteins encoded by corresponding ORFs in the CFA/I fimbrial operon. These proteins underlie the functional similarities between the CS1 and CFA/I fimbrial systems, allowing heterologous expression of their respective subunits.

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