Family of class 1 integrons related to In4 from Tn1696

The class I integron In28, found in the multidrug resistance transposon Tn1 403, was found to be located in the res site of the backbone transposon and is flanked by a 5-bp direct duplication, indicating that it reached this p osition by transposition. In28 has a backbone structure related to that of In4, but has lost internal sequences, including the sul1 gene, due to an IS 6100-mediated deletion. In28 also lacks the partial copy of IS6100 found in In4 and contains different gene cassettes, blaP1, cmlA1, and aadA1. Int, t he class I integron found in the multidrug resistance plasmid R46, is also located in a putative res site and belongs to the In4 group. In1 has a shor ter internal deletion than In28 and has also lost one end. Additional integ rons with structures related to In4 were also found in databases, and most of them had also lost either one end or internal regions or both. Tn610 bel ongs to this group.