Replication Fork Collapse and Genome Instability in a Deoxycytidylate Deaminase Mutant

Ribonucleotide reductase (RNR) and deoxycytidylate deaminase (dCMP deaminase) are pivotal allosteric enzymes required to maintain adequate pools of deoxyribonucleoside triphosphates (dNTPs) for DNA synthesis and repair. Whereas RNR inhibition slows DNA replication and activates checkpoint responses, the effect of dCMP deaminase deficiency is largely unknown. Here, we report that deleting theSchizosaccharomyces pombe dcd1 + dCMP deaminase gene (SPBC2G2.13c) increases dCTP ∼30-fold and decreases dTTP ∼4-fold. In contrast to the robust growth of aSaccharomyces cerevisiae dcd1 Δ mutant, fission yeastdcd1 Δ cells delay cell cycle progression in early S phase and are sensitive to multiple DNA-damaging agents, indicating impaired DNA replication and repair. DNA content profiling ofdcd1 Δ cells differs from an RNR-deficient mutant. Dcd1 deficiency activates genome integrity checkpoints enforced by Rad3 (ATR), Cds1 (Chk2), and Chk1 and creates critical requirements for proteins involved in recovery from replication fork collapse, including the γH2AX-binding protein Brc1 and Mus81 Holliday junction resolvase. These effects correlate with increased nuclear foci of the single-stranded DNA binding protein RPA and the homologous recombination repair protein Rad52. Moreover, Brc1 suppresses spontaneous mutagenesis indcd1 Δ cells. We propose that replication forks stall and collapse indcd1 Δ cells, burdening DNA damage and checkpoint responses to maintain genome integrity.