Replication stress can be produced by various exogenous or endogenous factors that perturb the movement of replication forks. To overcome the stress, the cells provoke the DNA replication checkpoint to mobilize several pathways to protect the forks, maintain genome integrity, and promote cell survival. Defects in the replication checkpoint cause forks collapse, leading to chromosomal DNA damage or cell death. Although the replication checkpoint is crucial for genome integrity in all eukaryotes, the underlying mechanisms remains to be fully understood. To investigate the mechanisms of DRC, hydroxyurea (HU), an established inhibitor of ribonucleotide reductase (RNR), has been widely used in laboratories as an inducer of the replication stress. It depletes dNTP pools, slows down the movement of DNA polymerase at the forks, and thus activates the replication checkpoint pathway in yeasts and in mammalian cells. Unfortunately, HU also targets other cellular components, which may complicate the studies, leading to ambiguous description of the checkpoint mechanisms. The purpose of this study is to find an RNR inhibitor that produces the replication stress more specifically than HU in the fission yeast Schizosaccharomyces pombe. We examined three RNR inhibitors guanazole, triapine, and gemcitabine under several experimental conditions that are commonly used in the laboratories for checkpoint studies. We found that among the three drugs, guanazole and triapine produce the replication stress more specifically than HU under the chronic drug exposure conditions such as the spot assay. Under acute drug treatment conditions, however, guanazole and triapine cause other cellular stresses more significantly than HU. Therefore, using guanazole and triapine in chronic drug exposure conditions and HU in acute treatment can produce a specific replication stress under various experimental conditions and thus benefits the checkpoint studies in S. pombe and possibly the research in other model systems
