Abstract
Attempts were made to isolate open and covalently closed circular DNA from strains containing the IncJ plasmids. All of the methods tried were unsuccessful. It was shown that the IncJ plasmid R391 can integrate into the Escherichia coli K12 chromosome and can mobilize chromosomal markers from a single origin in an orientated manner. Evidently, the IncJ plasmids are integrated in the chromosome for most, if not all, of their existence; this explains the inability to isolate plasmid DNA from strains containing them.