Organization of tn2610 containing two transposition modules.

Transposon Tn2610, found in a conjugative plasmid from an Escherichia coli isolate recovered at a hospital in Chiba, Japan, in 1975, was completely sequenced. Tn2610 is 23,883 bp long and is bracketed by two transposition modules, a Tn1721-like module and a Tn21-derived module, which correspond, respectively, to the long inverted repeats IRa and IRb previously described for this transposon. Although both tnpA genes are intact, only that in the Tn21-derived module (IRb) functions in the transposition, while that in the Tn1721-derived module (IRa) cannot recognize the 38-bp imperfect repeat at the end of the IRb element. Both tnpR and res are present in IRa, while the tnpR gene of IRb is interrupted by the insertion of an IS26 insertion element. The intervening region, between the res site of the Tn1721 module and IS26, carries multiple integron-associated resistance genes within a Tn21 backbone, including a region identical to that found in the genome of Salmonella enterica serovar Typhimurium DT104. These findings suggest that Tn2610 originated from Tn1721 and Tn21, with extensive recombination events with other elements which have resulted in a complex mosaic structure.