Control of replication origin density and firing time in Xenopus egg extracts: role of a caffeine-sensitive, ATR-dependent checkpoint.
A strict control of replication origin density and firing time is essential to chromosomal stability. Replication origins in early frog embryos are located at apparently random sequences, are spaced at close ( approximately 10-kb) intervals, and are activated in clusters that fire at different times throughout a very brief S phase. Using molecular combing of DNA from sperm nuclei replicating in Xenopus egg extracts, we show that the temporal order of origin firing can be modulated by the nucleocytoplasmic ratio and the checkpoint-abrogating agent caffeine in the absence of external challenge. Increasing the concentration of nuclei in the extract increases S phase length. Contrary to a previous interpretation, this does not result from a change in local origin spacing but from a spreading of the time over which distinct origin clusters fire and from a decrease in replication fork velocity. Caffeine addition or ATR inhibition with a specific neutralizing antibody increases origin firing early in S phase, suggesting that a checkpoint controls the time of origin firing during unperturbed S phase. Furthermore, fork progression is impaired when excess forks are assembled after caffeine treatment. We also show that caffeine allows more early origin firing with low levels of aphidicolin treatment but not higher levels. We propose that a caffeine-sensitive, ATR-dependent checkpoint adjusts the frequency of initiation to the supply of replication factors and optimizes fork density for safe and efficient chromosomal replication during normal S phase.
Marheineke
Kathrin
K
Laboratoire de Génétique Moléculaire, UMR 8541, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris Cedex 05, France.
Hyrien
Olivier
O
D000818:Animals; D016590:Aphidicolin / Q000494:pharmacology; D064007:Ataxia Telangiectasia Mutated Proteins; D002110:Caffeine / Q000494:pharmacology*; D018797:Cell Cycle Proteins / Q000378:metabolism / Q000502:physiology*; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D004261:DNA Replication / Q000187:drug effects; D004587:Electrophoresis, Agar Gel; D004625:Embryo, Nonmammalian / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008297:Male; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism / Q000502:physiology*; D018741:Replication Origin; D016196:S Phase; D013094:Spermatozoa / Q000378:metabolism; D013997:Time Factors; D014981:Xenopus; D029867:Xenopus Proteins
D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D010726:Phosphodiesterase Inhibitors; D029867:Xenopus Proteins; D016590:Aphidicolin; D002110:Caffeine; D004247:DNA; C576898:Atr protein, Xenopus; D064007:Ataxia Telangiectasia Mutated Proteins; D017346:Protein Serine-Threonine Kinases