The inactivation of tet genes on a plasmid by the duplication of one inverted repeat of a transposon-like structure which itself mediates tetracycline resistance

Derivatives of a naturally occurring IncFII tetracycline resistance plasmid (pUB889) which are unable to confer resistance to tetracycline, and which were isolated from both partners of a married couple, have been examined. The tet genes are part of a transposon-like structure which is closely related to, but distinct from, Tn10. The lesions in the two plasmids examined are identical and involve the insertion of a nucleotide sequence of 0.9 × 10⁶ within the region encoding tetracycline resistance, expression of which is lost as a consequence. The extra DNA is homologous with the nucleotide sequence, a pair of which form the inverted repeats of the Tn10-like structure. We conclude that this nucleotide sequence comprises an IS element. The epidemiological implications arising from the origin of these plasmids are discussed.