Genes for biosynthesis and assembly of CS3 pili of CFA/II enterotoxigenic Escherichia coli: novel regulation of pilus production by bypassing an amber codon.

The complete nucleotide sequence of the 4746bp HindIII fragment encoding the genes for the biosynthesis and assembly of CS3 pili has been determined. By site-directed mutagenesis in conjunction with analysis of the plasmid-encoded proteins in minicells, the actual reading frames for the various products have been determined. This demonstrated that the genes for four of the proteins (63 kD, 48 kD, 33 kD, and 20 kD in size) are encoded entirely within the same open reading frame as a fifth protein (104 kD). However, for synthesis of this latter protein, suppression or readthrough of an internal amber codon is required. Termination at this codon is also necessary for synthesis of the former proteins. Two further proteins are also encoded within the HindIII fragment: a 27 kD precursor of a periplasmic protein and the 17.5kD precursor of the major CS3 fimbrial subunit.