[High plasmid pBR322 multimerization in Escherichia coli B].

In bacteria E. coli B (wild) plasmid pBR322 is present as a set of circular supercoiled multimers formed of up to 20 head-to-tail linked monomers. The highest degree of plasmid multimerization was achieved when the bacteria were grown in a mineral-glucose medium based on 0.15 M phosphate buffer (pH 7.5). In E. coli K12 grown under similar conditions, plasmid pBR322 was mainly present as monomers. A kinetic experiment (initially only monomeric plasmids were present in the cells) has shown that multimerization of plasmids occurs step by step, that is, by means of interplasmid recombinations. Therefore, in E. coli B the efficiency of interplasmid recombinations is higher in comparison with E. coli K12. This result reflects some differences between basic systems of genetic recombination operating in these two lines of E. coli.