Extrachromosomal DNA in chloramphenicol resistant myxococcus strains.

The presence of extrachromosomal DNA in strains of Myxococcus xanthus and M fulvus was examined by rate-zonal centrifugation of radioactively-labelled DNA in 'cleared lysates'. All the strains examined contained extrachromosomal DNA, with the exception of M. xanthus FBt. Chloramphenicol resistance is inducible in M. xanthus FBt. 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 Escherichia coli with myxococci, R factor-mediated chloramphenicol resistance can be introduced into the latter. Evidence of extra chromosomal DNA in a derivative of M. xanthus with chloramphenicol resistance from R factor RI. 19 unique to the chloramphenicol strain, was obtained. By using a double-labelling technique, several chloramphenicol-resistant strains of M. fulvus 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.