RNA Turnover

Nuclear rRNA precursors and cytoplasmic rRNA formed by RNA polymerase I account for some 70+% of mammalian cell RNA and 35-40% of total RNA synthesis. The products of RNA polymerase II, hnRNA and mRNA, comprise some 10% of the total RNA and 55-60% of total synthesis. The hnRNA and mRNA transcripts are unstable and have half-lives in higher eukaryotes of about 24 h, compared with an average half-life in bacteria of 90 s. Pre-tRNAs, synthesized along with 5 S rRNA by RNA polymerase III, are relatively stable, with an average half-life of 4-6 days. 

A number of exoribonucleases and endoribonucleases with varying specificities degrade RNA in the cytoplasm to nucleotides. Unusual nucleotides derived from rRNA and tRNA are excreted, whereas the four common nucleotides (A, G, C, and U) are readily recycled. 

In DNA replication of the lagging strand the following enzymes are used  

Using the number for the enzymes, select the correct sequence for the DNA replication process. 

All the following statements are true about DNA replication, except 

---

Digestion of endogenous nucleic acids (cell death, RNA turnover)... 

Raghow, R., Regulation of messenger RNA turnover in eukaryotes. Trends Biochem. Sci. 12 3358-3360, 1987. 

Volume 447. RNA Turnover in Bacteria, Archaea and Organelles Edited by Lynne E. Maquat and Cecilia M. Arraiano... 

Volume 448. RNA Turnover in Eukaryotes Nucleases, Pathways and Analysis of mRNA Decay... 

Steady-state levels of RNA can be controlled by balancing rates of synthesis and degradation. Both endoribonucleases and exoribonucleases play a role in RNA degradation pathways. Control of transferrin-receptor mRNA levels by cytoplasmic aconitase/IRE-BP activities is a classic example of regulation of RNA turnover rates in response to physiological signals. 

The steady-state level of RNA in the cell is controlled both by RNA synthesis rates and by RNA turnover. What answer best explains how an alternate polyadenylation site might be utilized to control RNA stability ... 

RNA turnover, e.g. turnover of the -CCA sequence of tRNA, modulation of poly(A) tract of mRNA... 

To check if RNA turnover was influencing our assays, pulse-chase experiments were carried out (Table I, column II). When 5 min pulselabeling periods were followed by a 5 min chase, the amount of radiolabel in psbD-psbC transcripts declines significantly (13% to 53%). PsbD-psbC RNAs transcribed in plastids of 8 day-old dark-grown plants were less... 

SRIRAM, G. and TAYLOR, M.W. Purineless death Ribosomal RNA turnover in a purine-starved ade mutant of Chinese hamster cells. J. Biol. Chem. (1977), 2, 5350-5555. 

When normocaloric nutrition has been reached in anorexia and enteritis, a peak excretion can be observed. This obviously overshooting RNA turnover in an early anabolic state might tentatively be interpreted as to reflect an extremely active protein synthesis in the phase of regeneration of essential enzymes. In analogy the peak... 

In continuous anabolism the RNA-turnover is stabilized on an intermediate level as documented in anorexia and hypothyreoidism. The enteritis patient was dismissed before we could explore this stage. 

The excretion of "one way" methylated nucleosides reflects the activity of RNA turnover. Therefore it can be interpreted as a new indicator of catabolism-anabolism. 

K. Nakano, T. Nakao, K. H. Schram, W. M. Hammargren, T. D. Mcclure, M. Katz, et al, Urinary-Excretion of Modified Nucleosides as Biological Marker of RNA Turnover in Patients with Cancer and Aids, Clin. Chim. Acta, 1993, 218(2), 169-183. 

The first was thought to result from an Increase in enzyme concentration on "switching off" non-translatable RNA synthesis and the second an increase in substrate concentration as a result of non-translatable RNA "turnover". Depending on the nutritional environment and the precise conditions of growth it was suggested that these effects could vary in their relative importance in contributing towards increased exoprotein formation in the post-exponential phase of the growth cycle. 

There remained, however, one possibility to save the RNA synthesis for protein hypothesis. Since there is evidence that part, at least, of the cellular RNA is metabolically unstable, the net increase in RNA is by no means a measure of the amount synthesized and so one should measure RNA turnover rather than net synthesis. Here, however, the results were even more discouraging. While in actively growing and dividing cells the turnover data simply reflected those on the net increase in RNA, and hence showed a positive correlation with protein synthesis, in nondividing cells, which were actively engaged in protein synthesis, RNA turnover was generally very small compared with the rate of protein formation. 

It is interesting to note that all the known direct effects of protein S3Tithesis on RNA turnover are concerned rather specifically with individual nucleotides, rather than with RNA synthesis or turnover as a whole. It is possible that all of the synthesis and turnover of RNA observed in vivo in connection with protein synthesis is of this type, at least in the cytoplasm. Although there seems to be a limited turnover of the microsomal RNA 393), there is no certainty on this point. It is possible, for example, that the labeling observed in vivo is due to synthesis of complete new particles, rather than to the turnover of a small fraction of the RNA of all the particles. At any rate, it is quite certain that microsomal turnover is small compared with that of the cytoplasmic RNA. 

---