Broad host range IncP group plasmids are important contributors to horizontal gene transfer and the spread of antimicrobial resistance. They are comprised of six main lineages (a, b, g, e, d and h) based on phylogenetic differences in trfA. They share a similar “backbone” of ~40 kb suggesting that all have evolved from a common ancestor. The backbone contains relatively few restriction endonuclease cleavage sites although pronounced clustering of such sites reflects the insertion of foreign DNA (“genetic load”). The regions between Tra1-Tra2 and oriV-trfA are the main foci for attraction of genetic loads in the best characterised plasmids (IncPb). The diversity observed among the IncPb plasmids is poorly characterised among the other IncP plasmids.

R702 and R938 are IncPa plasmids isolated from clinical sources in the 1970s. R702 (Km^R, Tc^R, Sm^R, Su^R and Hg^R) and R938 (Km^R, Tc^R, Sm^R, Su^R, Hg^R, Ap^R and Cm^R) encode a range of antimicrobial resistance genes. These genes are thought to be associated with transposable elements embedded within the plasmids. Each plasmid contains a genetic load within the oriV-trfA region but not in the Tra1-Tra2 region. Both R702 and R938 plasmids contain an additional genetic element in an unusual site within the plasmid backbone i.e., R702 in kleD and R938 adjacent to kfrA. The four genetic elements appear to be intact transposable elements that belong to the Tn3-family.

R702 carries a novel cryptic transposon within the oriV-trfA region with a unique module of genes encoding proteins of unknown functions. A Tn21-like element is present in the kleD gene of R702 and in the oriV-trfA region of R938. The other element in R938 contains a nested transposon that appears to have evolved by multiple insertion and deletion events. These findings reveal novel attributes of the structure and evolution of IncPa plasmids and their transposable elements.