Caffeine can override the S-M checkpoint in fission yeast

The replication checkpoint (or 'S-M checkpoint') control prevents progressi on into mitosis when DNA replication is incomplete. Caffeine has been known for some time to have the capacity to override the S-M checkpoint in anima l cells, We show here that caffeine also disrupts the S-M checkpoint in the fission yeast Schizosaccharomyces pombe, By contrast, no comparable effect s of caffeine on the S, pombe DNA damage checkpoint were seen, S, pombe cel ls arrested in early S phase and then exposed to caffeine lost viability ra pidly as they attempted to enter mitosis, which was accompanied by tyrosine dephosphorylation of Cdc2, Despite this, the caffeine-induced loss of viab ility was not blocked in a temperature-sensitive cdc2 mutant incubated at t he restrictive temperature, although catastrophic mitosis was prevented und er these conditions. This suggests that, in addition to S-M checkpoint cont rol, a caffeine- sensitive function may be important for maintenance of cel l viability during S phase arrest. The lethality of a combination of caffei ne with the DNA replication inhibitor hydroxyurea was suppressed by overexp ression of Cds1 or Chk1, protein kinases previously implicated in S-M check point control and recovery from S phase arrest, In addition, the same combi nation of drugs was specifically tolerated in cells overexpressing either o f two novel S, pombe genes isolated in a cDNA library screen. These finding s should allow further molecular investigation of the regulation of S phase arrest, and may provide a useful system with which to identify novel drugs that specifically abrogate the checkpoint control.