Analysis of pCERC7, a small antibiotic resistance plasmid from a commensal ST131 Escherichia coli, defines a diverse group of plasmids that include various segments adjacent to a multimer resolution site and encode the same NikA relaxase accessory protein enabling mobilisation.

The ampicillin resistance plasmid pCERC7, carrying transposon Tn2 with an IS4 insertion, was detected in the draft genome of a commensal Escherichia coli isolate. The genome data also revealed that this isolate belongs to ST131, clade B. pCERC7 is 9712bp comprised of a 3319bp backbone, Tn2::IS4 (6388bp) and 5bp of target site duplication, and was present at a copy number of 40. pCERC7 is related to several plasmids composed of only the backbone, or the backbone with the Tn2 insertion in the same position. These plasmids have been found previously in Escherichia coli or Salmonella enterica recovered in several different countries from as early as the 1970s. This group was named the NTP16 group after the best studied example. pCERC7 was annotated using available information about plasmids in this group and additional analyses. The backbone includes genes for RNA I and RNA II to initiate replication and the Tn2 interrupts a gene found here to encode a protein 66% identical to the Rom regulatory protein of ColE1. NTP16 family plasmids include a gene, previously designated mobA, that was found to encode a homologue (53% identical) of the NikA relaxase accessory protein of the conjugative IncI1 plasmid R64, which is known to bind to the R64 oriT. However, a nikB relaxase gene is not present, indicating that a relaxase must be supplied in trans for mobilisation by R64 to occur, as demonstrated previously for NTP16. Hence, MobA of NTP16 and relatives was renamed NikA. Upstream of nikA, we found a region closely related to the oriT of R64. pCERC7 and all members of the NTP16 family also include a multimer resolution site, nmr, similar to the cer site of ColE1. The backbone of the NTP16 family also includes genes for a demonstrated toxin-antitoxin system, LsoAB. Several more distantly related groups of plasmids that include a very closely related nmr-nikA-oriT segment (99.4-93.7% DNA identity) were identified in the GenBank non-redundant DNA database. All use an RNA I/RNA II-Rom system for replication initiation, but each contains a unique fragment adjacent to the nmr site. The segment of the NTP16/pCERC7 group that encodes the LsoAB toxin-antitoxin system is replaced by a different segment in other family groups. The point at which the sequences diverge is between the XerC and XerD sites of the dif site at one end of nmr, suggesting that the evolution of this broad group of plasmids involves XerC/XerD recombination.
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