A symmetrical model for polyoma virus DNA replication

Abstract

Replicating polyoma virus DNA (DNA II ∗) is known to consist of three-branched circular duplexes containing short single-stranded regions. Under the action of a nuclease from Neurospora crassa conidia, which is highly specific for single-stranded nucleic acids, DNA II ∗ undergoes two successive configurational changes without being fragmented, resulting in marked alterations of its sedimentation properties. Electron microscopic observations performed at intervals on the incubation mixture show that DNA II ∗ is first converted into ring-shaped structubes with one tail attached, while the total number of branched duplexes tends to decrease subsequently. The latter observation suggests that the final product of the reaction might be a non-branched linear duplex. Since the configurational changes described develop concurrently with the disappearance of the single-stranded regions which DNA II ∗ contains, it is proposed that two such regions exist in most replicating molecules. They would be located at the two branch points, but on different replicated branches. On the basis of these observations, it is suggested that each branch point actually represents a growing point where replicating chains are elongated by different mechanisms, in accordance with their opposite polarities.