In order to elucidate whether data about the fast regulation of DNA replication in dependence on oxygen supply and on a functioning protein synthesis, previously elaborated with Ehrlich ascites cells, are valid for human cells too, we repeated key experiments with CCRF-CEM and HeLa cells. The most important techniques employed were DNA fibre autoradiography and alkaline sedimentation analyses of growing (pulselabeled) daughter strand DNA. It was found that CCRF-CEM and HeLa cells responded to transient hypoxia and to transient inhibition of protein synthesis in an almost identical fashion. Scheduled replicon initiations were reversibly suppressed and the progress rates of replication forks, which were already active before the respective inhibitory conditions were established, were reversibly slowed down. The inclusion of the fork progress rate in the response differs from Ehrlich ascites cells, which respond only by suppressing initiation. Further circumstances of the fast oxygen dependent response, concerning the behaviour of ribonucleotide reductase and of the dNTP pools, revealed no significant differences among the three cell lines. The striking identity of the response of each of the cell lines to hypoxia and to inhibited protein synthesis prompts the suspicion that converging fast regulatory pathways act on the cellular replication machinery. The phenomena as such seem to be rather widespread among mammalian cells.
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