Cloning and sequencing of the pertussis toxin genes: operon structure and gene duplication

Pertussis toxin, a protein composed of five different subunits (S1, S2, S3, S4, and S5), is the major virulence factor of Bordetella pertussis. We have cloned and sequenced a DNA fragment of 4.7 kilobases that contains the genes coding for the five subunits. The genes are clustered within 3.2 kilobases in the following order: S1, S2, S4, S5, and S3. A sequence closely resembling Escherichia coli promoters is found only before the S1 gene, and a possible termination signal is present at the end of the S3 gene, which suggests that the pertussis toxin genes are organized in a single operon. A possible Shine-Dalgarno sequence is present before the S1 gene but not before the other four genes that 8-12 nucleotides upstream from the ATG codon show a new consensus sequence, 5'TCC(T)GG3', possibly involved in the regulation of translation. We have also found sequence homology between the S2 and S3 genes and their protein products indicating that gene duplication played a major role in the evolution of pertussis toxin.