Ultracentrifugal Evidence for a Ring Structure

By velocity sedimentation, in appropriate solvents, the presence of two discrete components can be demonstrated in preparations of the DNA of bacteriophage <jJX174. The major, faster-moving 81 and the slower 82 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 82 is the first degradation product of 81 , formed by scission of 8 1, without significant decrease in molecular weight. A subsequent chain scission in 82 , which occurs with equal likelihood, results in random fragmentation. These results are interpreted to mean that the 81 component is a covalently linked ring structure and the 82 component is the corresponding open-chain degradation product. Under certain conditions the S~ 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 <jJX-DNA ring.