Novel dimeric configurations from bacteriophage G4 replicative form DNA

The oligomeric fraction of the replicative form of phage G4 was prepared by sedimentation on three successive CsCl velocity gradients followed by resolution on CsCl-propidium diiodide equilibrium gradients and subfractions through the equilibrium gradients were examined by electron microscopy. The most frequent dimer species were the circular dimer, the singly linked catenane and the figure 8; these occurred in a ratio of 10:3:1. The high enrichment for dimers and other oligomers made possible the observation and the determination of the frequency of occurrence of a number of minor species, some of them of novel configuration. These are (a) dimers similar to figure 8s except containing long, apparently four-stranded junctions common to the two halves (theta forms); (2) dimers similar to those in (1) except that the long junctions separate the two halves (dumbbell forms); (3) multiply catenated dimers with apparent right-handed intertwines; and (4) dimers containing a knot. Theta forms cleaved by EcoRI were shown to be stable under conditions in which EcoRI-treated figure 8s were resolved by branch migration.