[Molecular size of plasmid R18 and its substituted variants in Escherichia coli cells]

R-substituted plasmids (R18Arg) obtained from plasmid R18 and capable to convert Arg- strains E coli to Arg+ strains at the expense of genetic material mobilized from P. aeruginosa chromosome are physically unstable and exist as molecules of different size. It was shown that the physical size of plasmid R18Arg is 3--5 megadaltons greater than those of plasmid R18.