ISCR Elements Are Key Players in IncA/C Plasmid Evolution

In the recent paper by Call et al. (1), the authors show that sequence analysis of their IncA/C plasmids revealed that antimicrobial resistance genes were grouped in four main regions in addition to the region that harbors the bla_CMY gene in an otherwise highly related plasmid backbone structure. The authors show that the floR resistance region is found in all three plasmids as well as in the pSN254 plasmid and is found in close proximity to what the authors (and also a previous paper [9]) call insA. The authors also note that another antibiotic resistance gene cluster includes the resistance gene aadA which is found in three of these plasmids and is always found adjacent to an insertion sequence termed insE (9). These two insertions, insE and insA, are typical of insertion sequences previously designated ISCR elements (6, 7). ISCR elements are closely related to an unusual family of insertion sequences called the IS91 family (6, 7). These elements are known to move by a process called rolling-circle replication, and a function of this process is the concomitant movement of additional sequences found upstream of their transposase genes (5). To be specific, insE is indeed ISCR2, and insF is ISCR16 (8). ISCR elements are now recognized as powerful antibiotic resistance gene capture and movement systems that are also capable of constructing extended clusters of antibiotic resistance genes on plasmids as well as on chromosomes (7). The IncA/C plasmid group is now known to be very widely distributed both geographically and in many species of bacteria because of the plasmids' promiscuous behavior (3, 4). The IncA/C reference plasmid, pRA1, isolated in 1971 from the fish pathogen Aeromonas hydrophila harbors ISCR2 (2), indicating that this element has been closely involved in recruiting antibiotic resistance genes with IncA/C plasmids for some time. ISCR2 is often found in close proximity to the sul2 gene and a truncated glmM gene as a conserved resistance array among IncA/C plasmids; ISCR2 is often called an ISVsa3-like IS element or simply orfA (2). Interestingly, a recent publication has indicated that the ICE element SXT shares a common ancestor with the IncA/C plasmids (2), and ISCR2 together with truncated copies of this element are always found as part as SXT, implicating ISCR2 in the construction of this element (6). The ISCR16 element appears to be a more recent addition to the IncA/C plasmids and has been found in only three plasmids so far and always adjacent to groEL or groES gene(s) (8). This also is very interesting as this element and the closely related elements ISCR3 and ISCR4 are also found adjacent to sections of groEL as well as resistance genes, which has allowed us to track the evolutionary history of this element (8). Therefore, it appears that ISCR elements are key players in the recent evolution of the IncA/C plasmid group. Furthermore, gene capture systems such as ISCR elements allied with the promiscuous nature of IncA/C plasmids provides a new and powerful combination for rapid dissemination of antibiotic resistance genes.