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Summary: Streptomyces coelicolor A3(2) and S. lividans 66, which lack chloramphenicol acetyltransferase, gave rise to chloramphenicol-sensitive (Cmls) variants spontaneously at frequencies of 0·5 to 2%. The fertility type of S. coelicolor in respect of the scP1 plasmid (SCP1+, SCP1− or NF) had no effect on chloramphenicol sensitivity or on the frequency at which Cmls variants arose.
Cmls isolates spontaneously reverted to CmlR at frequencies one to three orders of magnitude lower than the frequency with which Cmls strains arose from CmlR CmlR revertants obtained spontaneously from Cmls clones again produced Cmls isolates at the normal frequency of several per cent. Therefore, Cmls and CmlR are reversible phenotypes.
In crosses between marked Cml r and Cml s S. coelicolor strains, transfer of chloramphenicol resistance into the sensitive strain apparently occurred independently of chromosomal recombination. Mapping experiments excluded the possibility that segregation of a chromosomal locus determines Cml r versus Cml s phenotype. In crosses between scP1− strains, fertility was not significantly different in Cml R × Cml s , Cml r × Cml r and Cml s × Cml s combinations.
Covalently closed circular DNA from Cml s and Cml r strains of S. coelicolor was indistinguishable in molecular weight and restriction endonuclease cleavage pattern.
It is suggested that chloramphenicol resistance in S. coelicolor a3(2) is affected by some kind of transposable genetic element that may be capable of extra-chromosomal existence.