Controlled protein degradation regulates ribonucleotide reductase activity in proliferating mammalian cells during the normal cell cycle and in response to DNA damage and replication blocks.

Ribonucleotide reductase (RNR) plays a central role in the formation and control of the optimal levels of deoxyribonucleoside triphosphates, which are required for DNA replication and DNA repair processes. Mammalian RNRs are composed of two nonidentical subunits, proteins R1 and R2. The levels of the limiting R2 protein control overall RNR activity during the mammalian cell cycle, being undetectable in G(1) phase and increasing in S phase. We show that in proliferating mammalian cells, the transcription of the R2 gene, once activated in the beginning of S phase, reaches its maximum 6-7 h later and then declines. Surprisingly, DNA damage and replication blocks neither increase nor prolong the R2 promoter activity in S phase. Instead, the cell cycle activity of the mammalian enzyme is controlled by an S phase/DNA damage-specific stabilization of the R2 protein, which is effective until cells pass into mitosis.