Ribosome synthesis rate

Gourse, R.L., Gaal, T., Bartlett, M.S., Appleman, J.A., Ross, W. (1996) rRNA transcription and growth rate-dependent regulation of ribosome synthesis in Escherichia coli. Annu. Rev. Microbiol. 50, 645-677. 

Over 100 genes in E. coli participate in the synthesis if of the RNAs and proteins that constitute the enzymatic machinery for translation. The relevant gene products make up between 20% and 40% of the dry cell mass. In rapidly growing cells about 85% of the RNA is ribosomal, 10% is tRNA, and most of the remainder is mRNA. The various RNAs and proteins are produced according to need. For ribosomes and tRNAs, this results in a synthesis rate that is roughly proportional to the cell growth... 

It is known that the amount of rDNA does not vary dramatically with different synthesis rates in tissues which synthesize ribosomal RNA (Brown, 1966) (Table 6). 

In the reactions above M and P respectively denote messenger RNA and protein, while hm and np are the corresponding molecular counts Sm and Sp are the substrates out of which RNA and protein molecules are made of, Ym and Yp correspond to the products into which RNA and proteins are degraded, Um and kp are the RNA and protein effective synthesis rates (accounting for the abundance of substrate moiecuies as well as of polymerases or ribosomes), and finally ym and yp are the RNA and protein degradation rates. 

Microscopic examination of the mature neutrophils reveals two striking features a single multilobed nucleus and a dense, granular appearance of the cytoplasm (see Fig. 1.1a). The nucleus typically comprises two to four segments, and within this organelle the chromatin is coarsely clumped. Until recently, this abnormal chromatin structure was taken as evidence that the nucleus was transcriptionally inactive however, it is now appreciated that the mature neutrophil does perform active transcription ( 7.3), although rates of biosynthesis are somewhat lower than those observed in cells such as monocytes. There is no detectable nucleolus, so there can be only limited synthesis of ribosomal RNA in these cells. 

When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover, and different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations ... 

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).

Piepersberg W, Noseda V, Bock A (1979) Bacterial ribosomes with two ambiguity mutations effects of translational fidelity, on the response to aminoglycosides and on the rate of protein synthesis. Mol Gen Genet 171 23-34... 

The translation of the correctly modified mature mRNA by the ribosome is also subject to regulation. The regulatory site of translation is mainly at the initiation of translation. Further regulatory elements include the availability of mRNA for ribosomal protein biosynthesis, as well as the concentration of mRNA. The availabihty of mRNA can be controlled by, for example, sequence-specific protein binding to the mRNA. The concentration of a specific mRNA is determined by a balance between its rate of synthesis (i.e. transcription) and its rate of degradation by RNases. The stabihty of a mRNA against nucleolytic degradation is thus a further factor that can determine the extent of biosynthesis of a protein. 

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