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Electron Microscopic Evidence for Linear Insertion of Bacteriophage MU-1 in Lysogenic Bacteria

Temperate bacteriophage Mu-1 was used to generate a lysogenic derivative of the F′lac episome of Escherichia coli. Intact, covalently circular molecules of F′lac and lysogenic F′lac Mu+ deoxyribonucleic acid (DNA) were isolated and examined by electron microscopy. The mean contour lengths of F′lac and F′lac Mu+ molecules were 37.6 ± 0.4 μm and 53.2 ± 0.4 μm, respectively. The mean difference, 15.6 μm, is similar to the mean contour length of 12.9 ± 0.1 μm obtained for linear DNA molecules released by osmotic shock from mature phage Mu-1 virions. These results provide direct physical evidence that phage Mu-1 integrates by linear insertion of its genome into the DNA of lysogenic host bacteria. Chemical and physical analyses of phage Mu-1 DNA indicate that it is similar to E. coli DNA in respect of gross base composition, buoyant density, and melting temperature.