SUMMARY: A3(2) and 66, which lack chloramphenicol acetyltransferase, gave rise to chloramphenicol-sensitive (Cml) variants spontaneously at frequencies of 0.5 to 2%. The fertility type of in respect of the SCP1 plasmid (SCP1, SCP1 or NF) had no effect on chloramphenicol sensitivity or on the frequency at which Cml variants arose.

Cml isolates spontaneously reverted to Cml at frequencies one to three orders of magnitude lower than the frequency with which Cml strains arose from Cml Cml revertants obtained spontaneously from Cml clones again produced Cml isolates at the normal frequency of several per cent. Therefore, Cml and Cml are reversible phenotypes.

In crosses between marked Cml and Cml strains, transfer of chloramphenicol resistance into the sensitive strain apparently occurred indepen-dently of chromosomal recombination. Mapping experiments excluded the possi-bility that segregation of a chromosomal locus determines Cml versus Cml phenotype. In crosses between SCP1 strains, fertility was not significantly different in Cml x Cml, Cml x Cml and Cml x Cml combinations.

Covalently closed circular DNA from Cml and Cml strains of was indistinguishable in molecular weight and restriction endonuclease cleavage pattern.

It is suggested that chloramphenicol resistance in A3(2) is affected by some kind of transposable genetic element that may be capable of extra-chromosomal existence.


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