The complete 41 268 bp nucleotide sequence of the IncP-1b plasmid pBP136 from the human pathogen Bordetella 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-1b backbone, and 2 ORFs similar to the XF1596 and XF1597 genes with unknown function of the plant pathogen Xylella 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 the kle genes (kleAE) that have been reported to be important for the stability of IncP-1 plasmid in Pseudomonas 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 the kle genes respectively, and the replication-initiation protein, TrfA, were closely related to those of the IncP-1b 'R751 group'. These results suggest that (i) pBP136 without any apparent accessory genes diverged early from an ancestor of the present IncP-1b plasmids, especially those of the R751 group, and (ii) the kle genes might be incorporated independently into the backbone region of the IncP-1 plasmids for their stable maintenance in various host cells.
INTRODUCTIONThe bacterial incompatibility group P-1 (IncP-1) plasmids can replicate and be stably maintained in almost all Gramnegative bacteria, and these plasmids are very promiscuous as either antibiotic-resistance or xenobiotic-degradative plasmids (Adamczyk & Jagura-Burdzy, 2003;Dennis, 2005;Thomas, 2000;Thomas & Smith, 1987). From the 1970s, many new members of the IncP-1 group have been discovered in bacteria from aquatic and soil environments (pB3, Heuer et al., 2004; pADP-1, Martinez et al., 2001; pB10, Schlüter et al., 2003; pB8, Schlüter et al., 2005; pUO1, Sota et al., 2003; pB4, Tauch et al., 2003; pTB11, Tennstedt et al., 2005; pJP4, Trefault et al., 2004; pEST4011, Vedler et al., 2004). The complete genome sequences of these newly identified IncP-1 plasmids have been determined, and the sequence data revealed that these plasmids have IncP-1-specific backbone modules for their replication, stable inheritance and conjugative transfer which show high similarity to the corresponding modules of two wellcharacterized IncP-1 plasmids, the IncP-1a subgroup plasmid RP4 (Pansegrau et al., 1994) and the IncP-1b subgroup plasmid R751 (Thorsted et al., 1998). Their genomes have various mobile genetic elements (transposons and their remnants) that carry genes encoding antibiotic resistance or degradation of xenobiotic compounds. Most of these accessory genes are inserted in one or both of two specific regions of the backbone, e.g. the IncP-1b plasmid pUO1 contains two haloacetate-catabolic transposons in the oriV-trfA region and two mercury-resistance transposons in the trb-tra region (Sota et al., 2003). In general, IncP-...