Chloramphenicol-resistant Myxococcus virescens were obtained by infecting myxococci with Escherichia coli specialized transducing phage PI CM. The drug-resistant myxococci were phenotypically unstable. They contained more than one type of plasmid; these plasmids were not found in the parent strain. Chloramphenicol-resistant E. coli were obtained by transformation with either a fraction of myxococcal DNA containing the plasmids or with PI CM prophage DNA. These transformants contained plasmids. Escherichia coli transformed by DNA from the myxococci contained both PlCM and myxococcal genes. Individual transformant clones differed in the genetic make-up of their plasmids. Among the myxococcal genes expressed in these plasmid-harbouring E. coli strains were a capacity for self-transmissibility and a pattern of phage sensitivity characteristic of R factor incompatibility group W. Escherichia coli transformed with P1 CM prophage contained incomplete P1 CM genomes ; none of the chloramphenicol-resistant transformants produced P1 CM phage particles. The significance of these findings for an understanding of mechanisms for the generation of R factors is discussed.
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