2'-Deoxy-2'-azidocytidine inhibits the initiation of polyoma DNA synthesis

2'-Deoxy-2'-azidocytidine, a nucleoside analogue, inhibits polyoma DNA synthesis. The inhibition results in a 5.5-fold decrease in the amount of replicating intermediates, as detected by electron microscopy. The results indicate that replication is inhibited at an early step, possibly in the initiation of new rounds of replication. In contrast, the effect on the elongation of DNA chains appears to be less pronounced. As a result of the inhibition, polyoma DNA molecules at an early stage of replication accumulate in the nuclei. Upon incubation of such nuclei, DNA synthesis is induced in these molecules. The replication starts from the normal origin and elongation proceeds bidirectionally at a rate close to that observed during in vivo conditions. Analysis of polyoma DNA replication in nuclei isolated from control cells indicates that both initiation and termination of replication are impaired under in vitro conditions.