Ribosomes Need Straight A's to Sleep

Abstract

Like falling asleep and waking up, many biological processes in mammals cycle in a diurnal fashion. Now, Sinturel et al. demonstrate that diurnal size changes in the liver require eating during a mouse's normal awake time and that these size changes are controlled by a nuclear mechanism that modulates ribosome production.

Figure 1

Left: Mice, being nocturnal, are at their most active during the night, which is when they typically eat. During the active phase in night-time or ad libitum fed mice, the liver enlarges, accompanied by an increase in cellular volume caused by increased levels of mature ribosomes. Levels of small ribosomal subunit (SSU) ribosomal proteins are increased, and there is less excess 18S-E pre-rRNA, the last nuclear precursor to mature SSUs, relative to SSU ribosomal protein levels. A mechanism for clearing pre-rRNA that is not stoichiometrically associated with ribosomal proteins occurs via polyadenylation and degradation. Since there is less excess pre-rRNA relative to ribosomal proteins, the 18S-E pre-rRNA is polyadenylated and degraded less frequently. Right: During the rest phase, there are fewer SSU ribosomal proteins leading to increased excess 18S-E pre-rRNA that is then polyadenylated and degraded. This, in turn, decreases levels of mature ribosomes, shrinking the liver at the organ and cellular levels.