Chloramphenicol resistance-specifying plasmids from incompatibility groups P-l and C did not encode chloramphenicol acetyltransferase (CAT). Expression of resistance was inducible by subinhibitory concentrations of the drug. The mechanism of resistance was thought to be a cytoplasmic membrane-located barrier to the permeability of the drug into the cell. No evidence for the inactivation of the drug was obtained. In vitro polypeptide synthesis directed by ribosomes isolated from resistant and sensitive cells was equally sensitive to inhibition by chloramphenicol suggesting that a ribosomal mechanism was not involved. Spheroplasts expressed the same level of resistance as whole cells. Strains specifying intracellular CAT did not degrade chloramphenicol in the culture medium if they also carried a chloramphenicol resistance plasmid not specifying CAT.


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