The BHR plasmids typically have mosaic genomes including two distinct regions (Thomas, 2000). The broad-host-range (BHR) plasmids have been defined as those plasmids that can self-transfer themselves and can stably replicate and maintain in bacterial species from at least two subgroups within the Proteobacteria (e.g., between α- and β- Proteobacteria) (Szpirer et al., 1999 Sen et al., 2011). They are important members of the mobile gene pool, and are among the most important contributors to horizontal gene transfer between bacteria (Frost et al., 2005). Plasmids are extra-chromosomal self-replicating DNA elements within the microorganisms (Mela et al., 2008). Our study increases the available collection of complete genome sequences of BHR plasmids, and since pSFA231 is the only characterized PromA plasmid from China, our findings also enhance our understanding of the genetic diversity of this plasmid group in different parts of the world. Alternatively, they may also be accessory genes that were first acquired and then stayed as the plasmid diverged. Interestingly, a cluster of hypothetical orfs located between parA and traA of pSFA231 shows high similarity with the corresponding regions on pMOL98, pIPO2T, and pTer331, suggesting these hypothetical orfs may represent “essential” plasmid backbone genes for the PromA-β subgroup.
We propose to divide the PromA group into two subgroups, PromA-α (pMRAD02, pSB102) and PromA-β (pMOL98, pIPO2T, pSFA231, pTer331), based on the splits network analysis of the RepA protein. Nevertheless, phylogenetic divergence was found in specific gene products. Further comparative genomic analysis shows that pSFA231 shares the common backbone regions with the other PromA plasmids, i.e., genes involved in replication, maintenance and control, and conjugative transfer. Phylogenetic analysis grouped pSFA231 into the newly defined PromA plasmid family, which currently includes five members. Based on its complete sequence the plasmid has a size of 41.5 kb and codes for 50 putative open reading frames (orfs), 29 of which represent genes involved in replication, partitioning and transfer functions of the plasmid. A self-transmissible broad-host-range (BHR) plasmid pSFA231 was isolated from petroleum-contaminated sediment in Shen-fu wastewater irrigation zone, China, using the triparental mating exogenous plasmid capture method.
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