Enterotoxigenic Escherichia coli strains express fimbriae which mediate binding to intestinal mucosal cells. The F17 fimbriae mediate binding to N-acetylglucosamine-containing receptors present on calf intestinal mucosal cells. These fimbriae consist of F17-A subunit peptides. Analysis of the F17 gene cluster indicated that at least the F17-A, F17-C, F17-D, and F17-G genes are indispensable to obtain adhesive F17 fimbriae (unpublished data). Genetic evidence is presented that the F17-G protein, a minor fimbrial component, is required for the binding of the F17 fimbriae to the intestinal villi. The F17-G gene was cloned and sequenced. An open reading frame of 1,032 bp encoding a polypeptide of 344 amino acids, starting with a signal sequence of 22 residues, was localized. The F17-G mutant strain produced F17 fimbriae which were morphologically identical to the fimbriae purified from strains which contained the intact F17 gene cluster. However, this F17-G mutant could no longer adhere to calf villi. The F17-G locus was shown to act in trans: transformation of the F17-G mutant strain, still expressing the genes F17-A, F17-C, and F17-D, with a vector expressing the F17-G gene restored the binding activity of this mutant strain.

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The genetic determinant for production of the fimbrial F17 adhesive antigen was isolated from a bovine enterotoxigenic Escherichia coli strain. The F17-A gene, coding for the structural component of the F17 fimbrial adhesin, was cloned and sequenced. An open reading frame of 540 base pairs encoding a polypeptide of 180 amino acids, of which the NH2-terminal 21 residues are characteristic of a signal sequence, has been characterized. The mature protein lacks histidine, methionine, and tryptophan. A possible promoter and ribosome binding site as well as a possible site for termination of transcription are proposed. An important homology of the F17-A protein with fimA and papA fimbrial proteins was found. The N-terminal sequence of the mature F17-A pilin is extremely similar to the N-terminal sequence of the G fimbriae identified on human pyelonephritogenic E. coli strains.

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