Evolution of IncP-1α plasmids by acquisition of antibiotic and mercuric ion resistance transposons.

The aim of this study was to examine the relationship between the IncP-1α plasmid RP1 (representing RP4, RK2, R68, and R18) and two plasmids, R1033 and R934, that are known to be related to RP1. The region containing most of the antibiotic resistance genes in R1033 and R934 was mapped using polymerase chain reaction and sequenced. Both plasmids contained a copy of the class II transposon Tn1, but it was located at different sites between the position of Tn1 and the tet(A) determinant in RP1. Thus, Tn1 and the ampicillin resistance gene contained in it were acquired by the IncP1-α backbone on three separate occasions. In R1033, Tn1696 is located nearby, between Tn1 and the tet(A) determinant, and an IS186 insertion sequence was also found in this region. In R934, a defective class II transposon carrying a partial mercuric ion resistance (mer) module was found within the Tn1. These findings show that R1033 is not derived directly from RP1/RP4, refining the evolutionary pathways previously predicted for the IncP-1α plasmid family.