Turnover of ribosomal RNA in cells in culture

Disruption of Dcp1 leads to a Dcp2-dependent aberrant ribosome profiles in Aspergillus nidulans

There are multiple RNA degradation mechanisms in eukaryotes, key among these is mRNA decapping, which requires the Dcp1-Dcp2 complex. Decapping is involved in various processes including nonsense-mediated decay (NMD), a process by which aberrant transcripts with a premature termination codon are targeted for translational repression and rapid decay. NMD is ubiquitous throughout eukaryotes and the key factors involved are highly conserved, although many differences have evolved. We investigated the role of Aspergillus nidulans decapping factors in NMD and found that they are not required, unlike Saccharomyces cerevisiae. Intriguingly, we also observed that the disruption of one of the decapping factors, Dcp1, leads to an aberrant ribosome profile. Importantly this was not shared by mutations disrupting Dcp2, the catalytic component of the decapping complex. The aberrant profile is associated with the accumulation of a high proportion of 25S rRNA degradation intermediates. We identified the location of three rRNA cleavage sites and show that a mutation targeted to disrupt the catalytic domain of Dcp2 partially suppresses the aberrant profile of Δdcp1 strains. This suggests that in the absence of Dcp1, cleaved ribosomal components accumulate and Dcp2 may be directly involved in mediating these cleavage events. We discuss the implications of this.

RNA stability in terminally differentiating fibre cells of the ocular lens

During terminal differentiation of lens fibre cells all cytoplasmic organelles are degraded abruptly. This process eliminates light-scattering elements from the optical axis of the lens and thereby ensures the transparency of the tissue. With the breakdown of the nucleus, transcription ceases, but the degree to which extant RNA is translated in the anucleated cells is uncertain. Previous studies indicated that fibre cell mRNA is unusually stable. For example, full-length delta-crystallin transcripts have been detected in core fibres months after transcription in these cells ceased. In the present study, we used the embryonic chicken lens as a model to examine the fate of RNA in the period immediately before and after organelle degradation. We mapped the tissue distribution of ribosomal RNA (rRNA) using acridine orange staining, in situ hybridization, and direct visualization of ribosomes by electron microscopy. These experiments suggested that rRNA decayed in the anucleated core fibre cells with a half-life of approximately 2.5 days. Similarly, in situ hybridization analysis of polyadenylated transcripts, beta-actin, or GAPDH mRNA indicated that these sequences were not stable in the core fibre cells. However, in agreement with earlier findings, we detected a strong in situ hybridization signal for delta-crystallin in the lens core, many days after transcription had ceased. We used quantitative PCR to compare the levels of GAPDH, L14 and delta-crystallin transcripts in the core region during development. Surprisingly, all three mRNAs decayed with indistinguishable kinetics. We conclude that the persistent delta-crystallin hybridization signal was not evidence of an unusually stable mRNA but, rather, reflected the extraordinary initial abundance of this transcript. Taken together, our data indicate that the half-life of both mRNA and the protein synthetic machinery in the lens core is only a few days. Given that, in vertebrate lenses, nuclei in this region of the lens are degraded during embryonic development, protein synthesis in central lens fibre cells is probably completed well before birth.

Dual pathways for ribonucleic acid turnover in WI-38 but not in I-cell human diploid fibroblasts

The turnover rates of 3H-labeled 18S ribosomal ribonucleic acid (RNA), 28S ribosomal RNA, transfer RNA, and total cytoplasmic RNA were very similar in growing WI-38 diploid fibroblasts. The rate of turnover was at least twofold greater when cell growth stopped due to cell confluence, 3H irradiation, or treatment with 20 mM NaN3 or 2 mM NaF. In contrast, the rate of total 3H-protein turnover was the same in growing and nongrowing cells. Both RNA and protein turnovers were accelerated at least twofold in WI-38 cells deprived of serum, and this increase in turnover was inhibited by NH4Cl. These results are consistent with two pathways for RNA turnover, one of them being nonlysosomal and the other being lysosome mediated (NH4Cl sensitive), as has been suggested for protein turnover. Also consistent with the notion of two pathways for RNA turnover were findings with I-cells, which are deficient for many lysosomal enzymes, and in which all RNA turnover was nonlysosomal (NH4Cl resistant).

Activities of cAMP-dependent protein kinase and enzymes of 2',5'-oligoadenylate metabolism in NIH 3T3 cells deepening into the resting state

The activity of cAMP-dependent protein kinase was found to increase continuously in the NIH 3T3 cells, deepening into the resting state. The increase correlated with intracellular level of heat-stable protein inhibitor of the protein kinase rather than with the cAMP content. The elevation of 2',5'-oligo(A) synthetase activity and the decrease in 2'-phosphodiesterase activity were also observed in the cells sinking into the resting state. The variations in enzyme activities were similar to those caused by the increase in the intracellular cAMP content described elsewhere. These results agree with the idea that the cAMP-dependent protein kinase is involved in the regulation of the enzymes of 2',5'-oligo(A) metabolism.

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.

Turnover of ribosomal 28S and 18S rRNA during rat liver regeneration

The turnover of 28S and 18S rRNA was studied in the course of 12 d after partial hepatectomy, including the proliferative (1st to 5th d) and post-proliferative (6th to 12th d) phases of liver regeneration. Turnover data, as the day-to-day rates of synthesis and degradation of 28S and 18S rRNA, were obtained by employing a suitable experimental procedure for the estimation of the increase of the amount of rRNA in the regenerating liver. It was found that 28S and 18S rRNA are accumulated into the cytoplasm and degraded at identical rates both in the proliferative and post-proliferative phases. The turnover of both rRNA moieties is markedly slower during the first 3 d of liver regeneration.

Turnover of ribosomes in regenerating rat liver

The turnover of ribosomes was studied in regenerating liver and the rates of ribosome degradation in the proliferative (1-5 days) and post-proliferative (6-12 days) phases after partial hepatectomy compared: Ribosome turnover during the first 3 days is slower, the half-life of ribosomes being approximately 300 hr. Ribosome synthesis is markedly activated--39% synthesis rate, which corresponds to 34% accumulation rate per day. The half-life of ribosomes during the second phase is about 107 hr. The rate of synthesis falls down to 19% the net gain of ribosomes being 3% per day.

Dual pathways for ribonucleic acid turnover in WI-38 but not in I-cell human diploid fibroblasts

The turnover rates of 3H-labeled 18S ribosomal ribonucleic acid (RNA), 28S ribosomal RNA, transfer RNA, and total cytoplasmic RNA were very similar in growing WI-38 diploid fibroblasts. The rate of turnover was at least twofold greater when cell growth stopped due to cell confluence, 3H irradiation, or treatment with 20 mM NaN3 or 2 mM NaF. In contrast, the rate of total 3H-protein turnover was the same in growing and nongrowing cells. Both RNA and protein turnovers were accelerated at least twofold in WI-38 cells deprived of serum, and this increase in turnover was inhibited by NH4Cl. These results are consistent with two pathways for RNA turnover, one of them being nonlysosomal and the other being lysosome mediated (NH4Cl sensitive), as has been suggested for protein turnover. Also consistent with the notion of two pathways for RNA turnover were findings with I-cells, which are deficient for many lysosomal enzymes, and in which all RNA turnover was nonlysosomal (NH4Cl resistant).