S6-haploinsufficiency activates the p53 tumor suppressor.

The capacity to detect and appropriately respond to many different stresses that interfere with functional homeostasis is essential for survival. Recent evidence suggests that the nucleolus, the site of ribosome biogenesis, plays a critical role in sensing and responding to both external and internal stresses. To understand these processes, we have recently used a genetically defined in vivo mouse model in which ribosome biogenesis could be manipulated during oogenesis and embryo development. In these mice ribosomal biosynthesis is impaired by a conditional deletion of one allele of the gene encoding 40S ribosomal protein S6. Embryos from these animals fail during gastrulation, apparently due to a p53-dependent checkpoint being triggered, rather than a deficit in translational capacity. These findings imply that molecular mechanisms have evolved during mammalian evolution to strongly guard against potential heterozygosity for ribosomal protein genes.