The structure of the DNA of bacteriophage φX174: III. Ultracentrifugal evidence for a ring structure

Abstract

By velocity sedimentation, in appropriate solvents, the presence of two discrete components can be demonstrated in preparations of the DNA of bacteriophage φX174. The major, faster-moving S_1 and the slower S_2 are present under conditions which exclude the possibility of hydrogen-bond formation. It can be shown, either by treatment with pancreatic deoxyribonuclease or by thermal inactivation, that S_2 is the first degradation product of S_1, formed by scission of S_1, without significant decrease in molecular weight. A subsequent chain scission in S_2, which occurs with equal likelihood, results in random fragmentation. These results are interpreted to mean that the S_1 component is a covalently linked ring structure and the S_2 component is the corresponding open-chain degradation product. Under certain conditions the S_2 component can be selectively degraded by E. coli phosphodiesterase. The digestion is not complete and there appears to be a single discontinuity, resistant to phosphodiesterase, present in the φX-DNA ring.