The supply of exogenous deoxyribonucleotides accelerates the speed of the replication fork in early S-phase

Earlier studies have established that the average speed of a replication fo rk is two to three times slower in early S-phase than in late S-phase and t hat the intracellular 2'-deoxyribonucleoside S-triphosphate pools grow duri ng S-phase. In this study, the effect of the exogenous 2'-deoxyribonucleosi de 5'-triphosphate (dNTP) supply on the average replication speed in a sync hronised population of human HeLa cells was tested. The speed of replicatio n fork movement was measured on extended DNA fibers labelled with 2'-deoxyt hymidine analogues 5-chloro-2'-deoxyuridine and 5-iodo-2'-deoxyuridine. We show that the introduction of exogenous dNTPs accelerates the replication p rocess at the beginning of DNA synthesis only. In late S-phase, the adminis tration of additional dNTPs has no effect on the speed of replication forks . The availability of 2'-deoxynucleotides seems to be a rate-limiting facto r for DNA replication during early S-phase.