Cytoplasmic localization of ribonucleotide reductase is essential for genome stability

Ribonucleotide reductase (RNR) is a major determinant of dNTP concentrations and its correct regulation is crucial for genome stability. Active RNR is cytoplasmic but the significance of this localization, away from DNA synthesis, is unclear, particularly as RNR is targeted to the nucleus after DNA damage occurs. We subverted the localization of fission yeast RNR by tagging the large (Cdc22 ^R1 ) and small (Suc22 ^R2 ) subunits with nuclear localization signal sequences. Directing RNR to the nucleus caused a slow S-phase and increased mutagenesis, in spite of predicted constitutive activation of RNR. Defective function of nuclear RNR is also suggested by the formation of Cdc22 ^R1 supramolecular filaments and defective RNR reoxidation. In contrast, constitutively nuclear Cdc22 or Suc22 has little effect on DNA damage repair and in fact suppresses rad3 Δ-sensitivity to DNA damaging agents. We suggest that there is an inadequate flux of substrate through nuclear RNR for S phase, but nuclear targeting of RNR may facilitate DNA damage repair under some circumstances.