We have characterized spSet1p, the Schizosaccharomyces pombe ortholog of the budding yeast histone H3 methyltransferase Set1p. SpSet1p catalyzes methylation of H3 at K4, in vivo and in vitro. Deleting spset1 partially affects telomeric and centromeric silencing. Strikingly, lack of spSet1p causes elongation of telomeres in wild-type cells and in most DNA damage checkpoint rad mutant cells, but not in cells lacking the ATM kinase Rad3 or its associated protein Rad26. Interestingly, spset1 deletion specifically causes a reduction in sensitivity to ultraviolet radiation of the PCNA-like checkpoint mutants hus1 and rad1, but not of cells devoid of Rad3. This partial suppression was not due to restoration of checkpoint function or to transcriptional induction of DNA repair genes. Moreover, spset1 allows recovery specifically of the crb2 checkpoint mutant upon treatment with the replication inhibitor hydroxyurea but not upon UV irradiation. Nevertheless, the pathway induced in spset1 cells cannot substitute for the Mus81/Rqh1 DNA damage tolerance pathway. Our results suggest that SpSet1p and the ATM kinase Rad3 function in a common genetic pathway linking chromatin to telomere length regulation and DNA repair. (C) 2003 Elsevier Science Ltd. All rights reserved.