In two unrelated plasmids of incompatibility groups FII and N the gene for the SHV-1 beta-lactamase exists as part of a transposable element of molecular weight 9.5 megadaltons. This transposon has moved onto plasmids of at least three incompatibility groups; PI, Ialpha and J. This confirms the suggestion that the recent spread of the SHV-1 beta-lactamase has been associated with the transposition of the genetic determinant of this enzyme between unrelated plasmids.

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Attempts were made to isolate open and covalently closed circular D N A 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. It is proposed that the IncJ plasmids are integrated in the chromosome for most, if not all, of their existence and this explains the inability to isolate plasmid D N A from strains containing them.

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