Fission yeast chk1 mutants show distinct responses to different types of DNA damaging treatments

Background: Chk1 kinase is activated by phosphorylation at serine-345 by Rad3 checkpoint kinase and is required for DNA damage checkpoint in late S and G2 phase of S. pombe cell cycle. We studied the ability of two chk1 mutants, chk1-1 and chk1-2, to undergo phosphorylation and to delay cell cycle progression in response to different types of DNA lesions. Results: Both the Chk1-1 and Chk1-2 mutant proteins are phosphorylated to various extents when DNA is damaged in early G2 phase of cell cycle by either UV irradiation or gamma irradiation. However, chk1-2 mutant does not delay cell cycle progression in a dose dependent manner specifically upon gamma irradiations. This defect is not associated with an important loss of survival. Furthermore, both chk1 mutants survive to Camptothecin treatment despite undetectable Chk1-1 or Chk1-2 phosphorylated forms. We show that both mutant proteins are not phosphorylated in cds1 devoid cells treated with ribonucleotide reductase inhibitor hydroxyurea or when the replisome is affected by a thermosensitive mutation in DNA polymerase δ. This inability is associated with the loss of checkpoint function. We found that an increased level of Crb2/Rhp9 protein specifically complements the defect of the chk1-1 mutant allowing Chk1-1 phosphorylation upon treatment with hydroxyurea of dcds1 cells. Conclusions: Mutants chk1-1 and chk1-2 behave differently according to the type of lesion generated on DNA.