The stability of ribonucleotide reductase is modulated by the E3 ubiquitin ligase PRL1

The evolutionarily conserved ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides (dNTPs), which are the building blocks for both DNA replication and DNA repair. Therefore, RNR is tightly regulated in all organisms. But it is still unknown how plants modulate RNR at post-translational level. The protein kinase ATR is a central regulator of replication stress responses. Through an activation tagging-based genetic screen, we found that overexpression of TSO2, a small subunit of the RNR, suppresses the hypersensitivity of Arabidopsis atr mutant to replication stress. Interestingly, TSO2 interacts with PRL1, which forms the Cullin4-based E3 ubiquitin ligase CRL4PRL1 and is negatively regulated by ATR. Further studies reveal that CRL4PRL1 ubiquitinates TSO2 at the lysine 295 and promotes its degradation via the 26S proteasome. Genetically, tso2 is epistatic to prl1 in response to replication stress. These results demonstrate that ATR inhibits PRL1 to enhance the stability of TSO2, not only providing novel insights into replication stress responses but also revealing new mechanisms of RNR, ATR, and PRL1.