RECONSTITUTION OF A MEC1-INDEPENDENT CHECKPOINT IN YEAST BY EXPRESSION OF A NOVEL HUMAN FORK HEAD CDNA

A novel human cDNA, CHES1 (checkpoint suppressor 1), has been isolated by suppression of the mec1-1 checkpoint mutation in Saccharomyces cer evisiae. CHES1 suppresses a number of DNA damage-activated checkpoint mutations in S. cerevisiae, including mec1, rad9, rad24, dun1, and rad 53. CHES1 suppression of sensitivity to DNA damage is specific for che ckpoint-defective strains, in contrast to DNA repair-defective strains . Presence of CHES1 but not a control vector resulted in G(2) delay af ter UV irradiation in checkpoint-defective strains, with kinetics, nuc lear morphology, and cycloheximide resistance similar to those of a wi ld-type strain. CHES1 can also suppress the lethality, UV sensitivity, and G(2) checkpoint defect of a mec1 null mutation. In contrast to th is activity, CHES1 had no measurable effect on the replication checkpo int as assayed by hydroxyurea sensitivity of a mec1 strain. Sequence a nalysis demonstrates that CHES1 is a hovel member of the fork head/Win ged Helix family of transcription factors. Suppression of the checkpoi nt-defective phenotype requires a 200-amino-acid domain in the carboxy terminus of the protein which is distinct from the DNA binding site. Analysis of CHES1 activity is most consistent with activation of an al ternative MEC1-independent checkpoint pathway in budding yeast.