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Abstract
BackgroundCheckpoint mechanisms prevent cell cycle transitions until previous events have been completed or damaged DNA has been repaired. In fission yeast, checkpoint mechanisms are known to regulate entry into mitosis, but so far no checkpoint inhibiting S phase entry has been identified.ResultsWe have studied the response of germinating Schizosaccharomyces pombe spores to UV irradiation in G1. When germinating spores are irradiated in early G1 phase, entry into S phase is delayed. We argue that the observed delay is caused by two separate mechanisms. The first takes place before entry into S phase, does not depend on the checkpoint proteins Rad3, Cds1 and Chk1 and is independent of Cdc2 phosphorylation. Furthermore, it is not dependent upon inhibiting the Cdc10-dependent transcription required for S phase entry, unlike a G1/S checkpoint described in budding yeast. We show that expression of Cdt1, a protein essential for initiation of DNA replication, is delayed upon UV irradiation. The second part of the delay occurs after entry into S phase and depends on Rad3 and Cds1 and is probably due to the intra-S checkpoint. If the germinating spores are irradiated in late G1, they enter S phase without delay and arrest in S phase, suggesting that the delay we observe upon UV irradiation in early G1 is not caused by nonspecific effects of UV irradiation.ConclusionsWe have studied the response of germinating S. pombe spores to UV irradiation in G1 and shown that S phase entry is delayed by a mechanism that is different from classical checkpoint responses. Our results point to a mechanism delaying expression of proteins required for S phase entry.
Highlights
Checkpoint mechanisms prevent cell cycle transitions until previous events have been completed or damaged DNA has been repaired
We have investigated the dependence of the delay on classical checkpoint proteins and showed that they are required for some but not all of the delay with 1C DNA content
We argue that the observed delay is caused by two separate mechanisms, the first taking place before entry into S phase, and the second in early S phase
Summary
Checkpoint mechanisms prevent cell cycle transitions until previous events have been completed or damaged DNA has been repaired. Checkpoint mechanisms are important for cell survival and genetic stability They prevent cell cycle transitions until previous events have been completed or damaged DNA has been repaired [1]. In fission yeast several checkpoint pathways monitor the status of the DNA and arrest the cell cycle in response to DNA damage or inhibition of DNA (page number not for citation purposes). Rad forms a complex with Rad and this association is required for activation of Rad kinase activity in response to DNA damage or replication arrest [17,18] The Rad, Rad and Hus proteins have similarities to PCNA, the sliding clamp of the replicative DNA polymerase, and the three proteins may form a similar ring-shaped structure [19,20,21]. Rad has similarities to all five subunits of replication factor C [22], a complex which loads PCNA onto chromatin
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