The 25·2 MDa tetracycline-resistance plasmid is not derived from the 24·5 MDa conjngative plasmid of Neisseria gonorrhoeae

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Abstract
High-level tetracycline resistance in strains of Neisseria gonorrhoeae is due to the presence of a 25·2 MDa conjugative plasmid. This plasmid has been shown to carry the streptococcal tetM determinant, and has been thought to have evolved from the 24·5 MDa conjugative plasmid found in N. gonorrhoeae. We have derived a physical map of the 25·2 MDa plasmid pUS100 using seven restriction endonucleases. Comparison of the physical map with the previously published physical map of the conjugative plasmid pLE2451 shows there to be no obvious similarity between the two plasmids. The location of the tetM determinant has been established, by Southern hybridization, confirming the restriction endonuclease map. This has also provided evidence that the transposition functions normally associated with the tetM determinant have been lost.