FIG. 9.

Nucleolar stress model of cell cycle arrest due to perturbation in ribosome biogenesis. Expression of mutant proteins such as Bop1Δ, exposure to chemical inhibitors of synthesis, and maturation of rRNA and other ribosome components in mammalian cells induce nucleolar stress, causing cell cycle arrest by triggering activation of the p53 pathway. p53 is required for sensing cellular stress caused by a variety of conditions, including DNA damage, replication defects, and gratuitous oncogene expression.