Plasmid pBP136 from Bordetella pertussis represents an ancestral form of IncP-1β plasmids without accessory mobile elements

Abstract

The complete 41 268 bp nucleotide sequence of the IncP-1βplasmid pBP136 from the human pathogenBordetella pertussis, the primary aetiological agent of whooping cough, was determined and analysed. This plasmid carried a total of 46 ORFs: 44 ORFs corresponding to the genes in the conserved IncP-1βbackbone, and 2 ORFs similar to theXF1596andXF1597genes with unknown function of the plant pathogenXylella fastidiosa. Interestingly, pBP136 had no accessory genes carrying genetic traits such as antibiotic or mercury resistance and/or xenobiotic degradation. Moreover, pBP136 had only two of theklegenes (kleAE) that have been reported to be important for the stability of IncP-1 plasmid inPseudomonas aeruginosa. Phylogenetic analysis of the Kle proteins revealed that the KleA and KleE of pBP136 were phylogenetically distant from those of the present IncP-1 plasmids. In contrast, IncC1 and KorC, encoded upstream and downstream of theklegenes respectively, and the replication-initiation protein, TrfA, were closely related to those of the IncP-1β‘R751 group’. These results suggest that (i) pBP136 without any apparent accessory genes diverged early from an ancestor of the present IncP-1βplasmids, especially those of the R751 group, and (ii) theklegenes might be incorporated independently into the backbone region of the IncP-1 plasmids for their stable maintenance in various host cells.