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 mum and 53.2 +/- 0.4 mum, respectively. The mean difference, 15.6 mum, is similar to the mean contour length of 12.9 +/- 0.1 mum 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.