THE INTEGRONS IN0, IN2, AND IN5 ARE DEFECTIVE TRANSPOSON DERIVATIVES

The class 1 integrons In0, In2, and In5, found in different locations in pVS1, Tn21, and pSCH884, have closely related structures. All three integrons contain an insertion sequence, IS1326, that is a new member of the IS21 family, IS1326 has caused deletions of adjacent 3'-conser ved segment and transposition module sequences, and all three integron s retain a complete copy of only one of four genes required for transp osition of related transposons and are thus defective transposon deriv atives. In2 contains an additional insertion sequence, IS1353, located within IS1326. IS1353 is a member of the IS3 family and appears to ha ve been acquired after the integron was inserted into an ancestral mer cury resistance transposon to create the ancestor of Tn21 and several other transposons that are close relatives of Tn21.