Characterization of the Escherichia coli AF/R1 pilus operon: novel genes necessary for transcriptional regulation and for pilus-mediated adherence.

We isolated the genetic determinant of AF/R1 pilus production in attaching/effacing Escherichia coli RDEC-1 and identified seven genes required for pilus expression and function. DNA sequence analysis of the structural subunit gene afrA corrected an error in the published sequence and extended homology with the F18 pilus subunit of pig edema E. coli strains. AfrB and AfrC, encoded downstream from AfrA, were required for pilus expression. AfrB was related to the usher protein PefC of Salmonella typhimurium plasmid-encoded fimbriae, and AfrC was related to PefD, a chaperone protein. AfrD and AfrE, encoded downstream from AfrC, were not necessary for the expression of AF/R1 pili but were required for ileal adherence as assayed by ileal brush border aggregation. Thus, the adhesive subunit of the AF/R1 pilus is distinct from the structural subunit, as is the case for Pap pili and type 1 pili. AfrD was related to FedE of the F18 fimbrial operon of the E. coli strain that causes edema disease in pigs. AfrE was a novel protein. AfrR and AfrS are encoded upstream from AfrA, in the opposite orientation. AfrR is related to the AraC family of transcriptional regulators, and AfrR and AfrS interact to function in a novel mode of transcriptional activation of afrA. AF/R1 pili mediate the adherence to Peyer's patch M cells, ileal mucosa, and colonic mucosa in a rabbit model of diarrhea caused by enteropathogenic E. coli. Our observations will facilitate the further study of the phenomena of M-cell adherence.