Replicative form II DNA of phi X174; resistance to exonucleolytic cleavage by E. coli DNA polymerase

The progeny replicative form II DNA isolated from E. coli cells infected with varphiX174 is resistant to the exonucleolytic activity associated with E. coli DNA polymerase. A limited endonucleolytic cleavage with micrococcal endonuclease renders the replicative form II molecule susceptible to the exonucleolytic activity associated with the E. coli DNA polymerase. These experiments suggest that the resistance of the replicative form II DNA to the exonucleolytic activity is a result of the structure of these molecules at their 5'-terminus.