Transposons Tn 1696 and Tn 21 and Their Integrons In4 and In2 Have Independent Origins

Abstract

The first 13.6 kb of the mercury and multidrug resistance transposon Tn 1696 , which includes the class 1 integron In4, has been sequenced. In4 is 8.33 kb long and contains the 5′-conserved segment (5′-CS) and 2.24 kb of the 3′-conserved segment (3′-CS) flanking four integrated cassettes. The 3′-CS region is followed by one full copy and an adjacent partial copy of the insertion sequence IS 6100 flanked, in inverse orientation, by two short segments (123 and 152 bp) from the outer right-hand end of class 1 integrons. This structure is representative of a distinct group of class 1 integrons that differs from In2, found in Tn 21 , and other related class 1 integrons. In4 does not include transposition genes but is bounded by characteristic 25-bp inverted repeats and flanked by a direct duplication of 5 bp of the target sequence, indicating that it was inserted by a transpositional mechanism. In4 lies between the resII and resI sites of a backbone mercury resistance transposon which is >99.5% identical to Tn 5036 . Although Tn 21 and Tn 1696 are both classified as members of the Tn 21 subfamily of the Tn 3 transposon family, the backbone mercury resistance transposons are only 79 to 96% identical. Tn 21 also contains a region of about 0.7 kb not found in Tn 1696 . The integrons In2 and In4 carrying the antibiotic resistance genes have been inserted at different locations into distinct ancestral mercury resistance transposons. Thus, Tn 21 and Tn 1696 have independent histories and origins. Other transposons (Tn 1403 and Tn 1412 ) that include a class 1 integron also have independent origins. In all except Tn 21 , the integron is located within the res region of the backbone transposon.