Recover)' of DNA synthesis after ultraviolet irradiation of xeroderma pigmentosum cells depends on excision repair and is blocked by caffeine

Normal human and xeroderma pigmentosum (XP, excision-defective group A) cells (both SV40-transformed) pulse-labeled with [3H]thym1d1ne at various times after irradiation with ultraviolet light showed a decline and recovery of both the molecular weights of newly synthesized DNA and the rates of syn-thesis per cell. At the same ultraviolet dose, both molecular weights and rates of synthesis were Inhibited more in XP than in normal cells. This indi-cates that excision repair plays a role in minimizing the inhibition of chain growth, possibly by excision of dimers ahead of the growing point. The abil-ity to synthesize normal-sized DNA recovered more rapidly than rates of syn-thesis in norma] cells, but both parameters recovered in phase in XP cells. During recovery in normal cells there are therefore fewer actively repli-cating clusters of replIcons because the single-strand breaks involved in the excision of dimers Inhibit repHcon initiation. XP cells have few excision repair events and therefore fewer breaks to interfere with Initiation, but chain growth is blocked by unexcised dimers. In both cell types recovery of the ability to synthesize normal-sized DNA was prevented by growing cells 1n caffeine after Irradiation, possibly because of competition between the DNA binding properties of caffeine and replication proteins. Our observations Imply that excision repair and semiconservative replica-tion Interact strongly 1n irradiated cells to produce a complex spectrum of changes 1n DNA replication which may be confused with parts of alterna-tive systems such as post-replication repair