Different turnover rates of brain ribosomal ribonucleic acids in male and female rats

Ribosome Assembly and Repair

Ribosomes synthesize protein in all cells. Maintaining both the correct number and composition of ribosomes is critical for protein homeostasis. To address this challenge, cells have evolved intricate quality control mechanisms during assembly to ensure that only correctly matured ribosomes are released into the translating pool. However, these assembly-associated quality control mechanisms do not deal with damage that arises during the ribosomes' exceptionally long lifetimes and might equally compromise their function or lead to reduced ribosome numbers. Recent research has revealed that ribosomes with damaged ribosomal proteins can be repaired by the release of the damaged protein, thereby ensuring ribosome integrity at a fraction of the energetic cost of producing new ribosomes, appropriate for stress conditions. In this article, we cover the types of ribosome damage known so far, and then we review the known repair mechanisms before surveying the literature for possible additional instances of repair.

Different Rates of Synthesis and Turnover of Ribosomal RNA in Rat Brain and Liver

The kinetics of in vivo labeling of cellular free UMP and nucleolar, nucleoplasmic, and cytoplasmic rRNA with [14C]orotate in rat brain and liver were investigated. Evaluation of the experimental data shows: (a) The rate of nucleolar precursors of ribosomal RNA (pre-rRNA) synthesis and the deduced rate of ribosome formation in brain is about fivefold lower than in liver and corresponds to 220-260 ribosomes/min/nucleus. (b) The lower rate of in vivo pre-rRNA synthesis is correlated with a lower activity of RNA polymerase I in isolated brain nuclei. (c) The half-lives of nucleolar rRNA in brain and liver are 210 and 60 min, respectively, thus showing a slower rate of processing of pre-rRNA in brain nucleoli. (d) The nucleo-cytoplasmic transport of ribosomes in brain is also markedly slower than in liver and reflects the lower rates of synthesis and processing of pre-rRNA. (e) Cytoplasmic ribosomes in brain and liver turn over with half-lives of about 6 and 4 days, respectively. It is concluded that the markedly lower rate of ribosome biogenesis in brain is specified mainly at the level of transcription of rRNA genes.

Minireview. Roles of turnover and repair of macromolecules and supramolecular structural components

Macromolecules and supramolecular structural components that are incorrectly synthesized or are damaged by radiation or by reactive chemicals are either repaired or selectively degraded and resynthesized. In addition, turnover rates for macromolecules and supramolecular structures can be elevated by alternation of fasting and feeding periods and can be influenced by metabolic regulatory mechanisms which are governed by steady-state concentrations of labile macromolecules.