The pertussis toxin liberation genes of Bordetella pertussis are transcriptionally linked to the pertussis toxin operon.

The DNA sequence of the pertussis toxin operon (ptx) of Bordetella pertussis predicts that transcription of the operon ends downstream from the ptxS3 gene at a possible stem-loop structure. Secretion of the assembled pertussis toxin into the culture medium required the expression of 8 genes arranged in an operon (ptl) and lying 55 bp downstream from the ptx and ptl operons are cotranscribed and coregulated by the P(TOX) promoter. Deletion of the 55-bp DNA region caused an increase in the amount of the ptl transcripts. It is likely that this DNA region is involved in regulation of the ptx-pti expression.