Nucleotide depletion reveals the impaired ribosome biogenesis checkpoint as a barrier against damage

Many oncogenes enhance nucleotide usage to increase ribosome content, replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint () and the damage response () have been implicated in p53 activation following nucleotide depletion. However, it is difficult to reconcile the two checkpoints operating together, as the induces p21-mediated G1 arrest, whereas the requires that cells enter S phase. Gradual inhibition of inosine monophosphate dehydrogenase (), an enzyme required for de novo synthesis, reveals a hierarchical organization of these two checkpoints. We find that the is the primary nucleotide sensor, but increased inhibition leads to p21 degradation, compromising -mediated G1 arrest and allowing S phase entry and activation. Disruption of the alone is sufficient to elicit the , which is strongly enhanced by inhibition, suggesting that the acts as a barrier against genomic instability. Hierarchical organization of p21-mediated G1 arrest and S-phase damage response signaling ensures genomic stability upon increasing inhibition of nucleotide synthesis enzyme.