SUMMARY: The presence of extrachromosomal DNA in strains of and was examined by rate-zonal centrifugation of radioactively-labelled DNA in ‘cleared lysates”. All the strains examined contained extrachromosomal DNA, with the exception of FB. Chloramphenicol resistance is inducible in FB. A peak of extrachromosomal DNA, containing covalently closed molecules, was found in one of the induced strains, implying that induction of chloramphenicol resistance is associated with the production of a plasmid.

By incubating R strains of with myxococci, R factor-mediated chloramphenicol resistance can be introduced into the latter. Evidence of extra chromosomal DNA in a derivative of with chloramphenicol resistance from R factor R1. 19 unique to the chloramphenicol strain, was obtained. By using a double-labelling technique, several chloramphenicol-resistant strains of M were examined. Evidence for a peak, unique for the chloramphenicol-resistant strain, was found in a strain with resistance derived from the R factor, S-a, but not from comparable strains with resistance derived from R factors R57b, R1. 19 and R478.


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