Heterogeneity of tetracycline resistance determinants

We have found that tetracycline resistance on different naturally occurring bacterial plasmids is encoded by more than one genetic determinant. Using restriction enzyme analyses and DNA-DNA hybridization to specific ³²P-labeled genetic probes, we can define at least four genetically distinct tetracycline resistance determiants: Class A (the determinant on prototype plasmid RP1), Class B (that on R222), and Class C (that on plasmid pSC101). At least one other determinant, encoded by plasmid RA1, belongs to none of these three groups and has been designated Class D. These genetic classes confirm phenotypic differences in expression of resistance to tetracycline and tetracycline analogs encoded by the different plasmids.