Extragenic suppressors of Schizosaccharomyces pombe rad9 mutations uncouple radioresistance and hydroxyurea sensitivity from cell cycle checkpoint control.

Schizosaccharomyces pombe cells that contain a mutation within rad9 are sensitive to ionizing radiation, UV light and hydroxyurea, relative to wild-type strains. In addition, the mutants are moderately hypomutable by UV and unable to delay initiation of mitosis after treatment with radiation or hydroxyurea. Three radioresistant derivatives of rad9::ura4 cells were isolated, and each contained a single unique extragenic suppressor responsible for the acquired resistance. The suppressor loci also conferred radioresistance upon cells containing rad9-192, which differs from rad9+ by a single base pair change. The suppressors additionally enhanced the radioresistance of cells containing rad3-136, a mutation that leads to phenotypes similar to those mediated by rad9::ura4. None of the derivatives of rad9::ura4 cells recovered the ability to delay cycling in G2 after exposure to ionizing radiation or UV light. All three suppressor derivatives, relative to the parental rad9::ura4 strain, also exhibited a moderate increase in resistance to the DNA replication inhibitor hydroxyurea without gaining the ability to stop progression into mitosis despite the inhibition of DNA synthesis. Results are discussed in terms of models to explain the putative role of rad9 and the suppressor genes in promoting radioresistance and mediating checkpoint controls responsive to DNA damage or incomplete DNA replication.