In general mammalian cells recover from DNA synthesis inhibition by ultraviolet light (u.v.) before most of the pyrimidine dimers have been removed from the genome. This is a complex phenomenon whose biological significance has not been fully assessed. In Chinese hamster V79 cells this recovery seems to be directly coupled to an enhanced rate of double-stranded DNA elongation. The presence of the DNA polymerase α inhibitor, aphidicolin, after u.v. irradiation produces two different responses. At low concentration, sufficient to inhibit 95 % of DNA replication but having no effect on excision repair, the drug has no effect on the recovery. This shows that ongoing replicative DNA synthesis is not required for recovery. At higher concentrations of aphidicolin, sufficient to block excision repair, the recovery phenomenon was prevented. The recovery was also prevented by actinomycin D at a concentration that inhibits 60 % of RNA synthesis. In quantitative autoradiography experiments in which previously irradiated cells were fused with unirradiated cells the nuclei of the latter exhibited a higher resistance to inhibition by u.v. than nuclei from non-fused cells. These results indicate that: (1) even the low repair rate exhibited by V79 cells (relative to human cells) is important for recovery; although most of the dimers remain in the V79 genome after recovery of DNA synthesis, either the removal of lesions from some important region of chromatin or the activity of the repair process itself is important for the recovery; (2) the recovery mechanism is induced and depends on RNA synthesis and the production of specific factors.
Finally, we have observed that cells previously treated with fluorodeoxyuridine become more resistant to inhibition by u.v. After irradiation these cells replicate DNA faster than untreated cells. Since it has been shown that this drug activates unused origins of replication in Chinese hamster cells, reducing the average replicon size, we assume that the acquired resistance has to do with the operation of a larger number of smaller replicons. This may also be the mechanism whereby recovery from inhibition occurs after u.v. irradiation.