Isolation and characterization of circular deoxyribonucleic acid obtained from lactose-fermenting Salmonella strains

Six lac elements originally contained in Salmonella strains were transferred to Escherichia coli WR3026. All of the six E. coli strains that received one of the lac elements were observed to contain supercoiled, circular deoxyribonucleic acid (DNA) when examined by the dye-buoyant density method. Segregants of each of these E. coli WR3026 strains that had lost the ability to utilize lactose, when examined in the same manner as the lactose-fermenting strains, were not observed to contain these supercoiled, circular DNA molecules. Thus the DNA of the lac elements is maintained in E. coli WR3026 in the supercoiled, circular form. Molecular weights of the supercoiled, circular molecules isolated from strains carrying the lac elements were determined by sucrose density gradient centrifugation to be 30 million to 56 million. The calculated number of copies per chromosome of the lac elements varied from 1.4 to 3.7, depending upon the particular lac element examined. Each of the elements was determined to have a guanine plus cytosine composition of 50%. All six of the E. coli WR3026 strains containing a transmissible lac element were tested with the E. coli male-specific phage, R-17, and the E. coli female-specific phage, phiII, and did not respond to either of these phages as do F-containing derivatives of E. coli K-12.