IS257-mediated cointegration in the evolution of a family of staphylococcal trimethoprim resistance plasmids

Analyses of the Staphylococcus epidermidis multiresistance plasmids pSK697 and pSK818 have revealed them to be closely related to the trimethoprim resistance plasmid pSK639, also isolated from S. epidermidis. pSK697 and pSK818 were found to contain a cointegrated copy of a second plasmid related to the S. epidermidis multidrug antiseptic and disinfectant resistance plasmid pSK108 and the S. aureus tetracycline resistance plasmid pT181, respectively. In contrast to pSK639, both plasmids were found to contain a third copy of IS257, such that the integrated plasmids in both cases are flanked by a copy of this element. This organization and the presence of duplicated sequences at the extremities of the integrated plasmids implicate IS257 in the formation of these cointegrate plasmids. Sequence analysis of the IS257 elements from these plasmids has provided insights into the probable mechanism of cointegration, viz., nonresolved replicative transposition of IS257.