Sequencing of the gene encoding the major pilin of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli and evidence that CFA/III is related to type IV pili.

The plasmid-encoded structural gene cofA necessary for the production of the major pilin subunit of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli was identified, and the nucleotide sequence of the gene was determined. cofA consists of 714 nucleotides encoding a 238-amino-acid protein (molecular weight of 25,309). CofA seems to be a precursor of CFA/III pilin, because the first 23 residues of the N-terminal amino acid sequence of the purified CFA/III pili coincided with the deduced amino acid sequence for residues 32 to 54 of CofA. Western blot (immunoblot) analysis of CofA also indicated its processing to form mature pilin in the presence of the downstream region of cofA. These results suggest that the major pilin of CFA/III pili is produced as a precursor form which is posttranslationally modified to the mature pilin and forms morphological pili after cleavage of the Gly-30-Met-31 junction, probably by a protease encoded by an as-yet-unknown gene located downstream of cofA. Interestingly, the N-terminal 30-amino-acid sequence of mature CFA/III shows the highest identity (76.7%) to TcpA pilin of Vibrio cholerae, which is a type IV class B pilin.