Effect of puromycin on DNA replication in Chinese hamster cells

We have used autoradiography to examine the effect of puromycin on DNA replication in Chinese hamster cells aligned by treatment with fluorodeoxy-uridine. In the absence of puromycin the patterns of replication are consistent with those obtained previously by others (Cairns, 1966; Huberman &; Riggs, 1968). In particular, replication occurs in tandem clusters of replicons but not all replicons in a cluster appear to be activated at the same time.Puromycin decreases the over-all rate of synthesis of DNA per cell, but does not inhibit chain elongation as visualized in autoradiograms. It is suggested that puromycin inhibits the initiation of replication of replicons not yet activated. Puromycin prevents the joining of short stretches of radioactive DNA into longer pieces. This may be due to the inability to activate a few late replicating units within a cluster of replicons.