RNA ribosomal synthesis

Most of the RNA made in the cell is ribosomal RNA. The large and small subunit RNAs are synthesized by RNA polymerase I. Ribosomal RNA is made in a specialized organelle, the nucleolus, which contains many copies of the rRNA genes, a correspondingly large number of RNA polymerase I molecules, and the cellular machinery that processes the primary transcripts into mature 

---

Thomas That is not absolutely true. In bacteria, if you change the energy charge you see a direct effect on translation and ribosome biosynthesis. As soon as ATP drops, ribosomal RNA synthesis decreases but not transcription of other genes. 

Figure 12.17 Sites at which insulin stimulates protein synthesis in a muscle. The sites are indicated by 0. Insulin has its anabolic effect on protein synthesis in muscle by affecb ng six processes or reactions (i) it inhibits protein degradab on in the muscle (ii) it sb mulates amino acid transport from the blood into the muscle (iii) it stimulates the inib ab on-reacb on of the pathway for protein synthesis, i.e. formab on of the complex (tRNA-amino acid-mRNA-ribosomal RNA) (iv) it increases the rate of mRNA synthesis, and therefore the number of mRNA molecules (v) it stimulates ribosomal RNA synthesis (vi) it sb mulates elongabon of the pepbde (see Chapter 20).

Ribosomal RNA. Quantitatively the most im portant RNA, making up 90% of that present in cells, is ribosomal RNA. Synthesis of rRNA must be rapid, for an E. coli cell produces 5-10 new ribosomes per second, or 2 x 104 molecules of RNA per generation. Bacterial ribosomes contain three pieces of RNA. 

The synthesis of ribosomal proteins is regulated at the level of translation. Certain ribosomal proteins bind to specific sites on the ribosomal RNAs or their own mRNAs. In the absence of the ribosomal RNAs, they bind to their own mRNAs, which inhibits their translation. This form of translational control regulates the rate of synthesis of ribosomal proteins so that it does not exceed the rate of ribosomal RNA synthesis. 

Fraser, R.S.S., Studies on messenger and ribosomal RNA synthesis in plant tissue cultures induced to undergo synchronous cell division, Eur. J. Biochem., 50, 529-537, 1975. 

The primary effect of these compounds on metabolism of sensitive fungi lies in their inhibition of ribosomal-RNA synthesis. The secondary effects of this inhibition is possible deaease in protein synthesis which would ultimately kill the fungus. 

Nucleolus. A spherical structure visible in the nucleus during interphase. The nucleolus is associated with a site on the chromosome that is involved in ribosomal RNA synthesis. 

Phenylamide fungicides selectively inhibit ribosomal RNA synthesis of sensitive Oomycete fungi by interference with the activity of the RNA polymerase I-template complex. 

In phenylamide-resistant strains, phenylamides are not able to inhibit ribosomal RNA synthesis, as is indicated by the inability of metalaxyl to interfere with uridine incorporation in a phenylamide-resistant strain of P. megasperma f. sp. megasperma obtained in the laboratory after mutagenic treatment (14) and a phenylamide-resistant strain of Phytophthora infestans (Table III) originating from a potato field (15). 

When nuclei are stained with certain dyes, one or more spherical structures called nucleoli (singular nucleolus) become visible. The nucleolus is the site of ribosomal RNA synthesis. Its high content of RNA makes it stain differently than the rest of the nucleus. 

Toyocamycin (314) is another adenosine derivative produced by Streptomyces toyocaensis [18]. This compound selectively inhibits the ribosomal RNA synthesis in fibroblasts. The synthesis of adenovirus-specific mRNA is also inhibited by 314 [18]. 

Jewett, M.C., Fritz, B.R, Timmerman, L.E., and Church, G.M. (2013) In vitro integration of ribosomal RNA synthesis, ribosome assembly, and translation. AIol. Syst. BioL, 9, 678. 

Table II is a partial list of a number of drugs that affect nucleolar function. For a more complete listing see Busch and Smetana (1970), Abelson and Penman (1975), and Brasch (1990). Nucleolar function can be perturbed directly through the action of antimetabolites on ribosomal RNA synthesis and processing or indirectly by affecting RNA polymerase Il-directed transcription and processing, or by effects on protein synthesis. By manipulating the level of exposure to Act D or a-amanitin, one can selectively inhibit either Pol I- or Pol Il-directed transcription.

Brown, D.D., Gurdon, J.B. Absence of ribosomal RNA synthesis in the anucleolate mutant of Xenopus laevis. Proc. nat. Acad. Sci. (Wash.) 51, 139-146 (1964)... 

Studies of the effects of cortisol on nucleic acids and protein of thymocytes in vivo and in vitro have shown interference with penetration of nucleic acids and protein precursors inhibition of RNA, DNA, and protein synthesis decreases in RNA polymerase activity and reduction in ribosomal RNA synthesis. Interestingly enough, these inhibiting effects of cortisol depend upon the presence of an energy source in the medium. 

Posttranscriptional and Transcriptional Regulation of Ribosomal RNA Synthesis in Adenovirus-Infected Cells... 

---