The control of protein synthesis

The control and expression of protein in eukaryotic cells is more complex than in bacteria. Research into this field is ongoing and, as such, is beyond the scope of this text. Many systems of control are known in bacteria and can be used to illustrate the type of control mechanisms and the importance of the environmental control of protein synthesis. The first and best documented example is that of the lactose (lac) operon is Escherichia coli. 

It has been established that some enzymes are produced only in the presence of their substrates and they are then said to be inducible. The quantity of such enzymes will vary considerably with changing environmental conditions. Other enzymes which are always detectable at constant levels, irrespective of nutrition and environmental conditions, are called constitutive enzymes. 

In the induction of enzymes of galactose metabolism in E. coli, three enzymes are involved -galactosidase (which catalyses the hydrolysis of the y -glycosidic bonds of lactose), galactose permease (which is responsible for transport of lactose across the cell membrane) and a third enzyme, A-protein, apparently not directly involved in galactose metabolism. The system has an environmental inducer, galactose, and in its presence the number of /)-galactosidase molecules rises from 5-10 to 10,000 within the cell. The addition of the inducer can increase the protein production in less than five minutes after its addition. Protein synthesis of these enzymes stops almost immediately in the absence of lactose. 

The molecular and genetic relationship between enzyme induction and repression was clarified by the genetic research of Jacob and Monod at the Pasteur Institute, Paris (see reference l7 ). Their classic work led them to develop the operon hypothesis for the control of protein synthesis in prokaryotes, which has since been verified by direct biochemical experiments. 

An operon thus consists of a series of functionally related structural genes which are turned on and off together, plus their regulatory gene, the operator. This overall 

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The answer to question (iv) is that although much is known about the control of protein synthesis, very little is known about the processes of degradation. 

Figure 8.16 The control of amino acid breakdown and protein synthesis in liver. The factors in regulation are as follows (i) the amino acid concentration in the blood regulates the rate of urea production (Chapter 10) (ii) the amino acid leucine, and the anabolic hormones increase the rate of protein synthesis. Mass action is a term used to describe the effect of concentration of substrate on the reaction rate. The control of protein synthesis is discussed in Chapter 20. Control by leucine has been studied primarily in muscle.

The most important difference between replication and transcription is that not all the DNA is used in the transcription. Usually, only small groups of genes are transcribed at any one time. Thus, the transcription of DNA is selective, turned on by specific regulatory sequences which indicate the beginning and ending of the region to be transcribed. The control of protein synthesis is discussed further in Section 5.7. 

These findings started a tremendous wave of research interest (a fashion) into the potential role of cytokinins in nucleic acid synthesis (and hence the control of protein synthesis) in plants. Looking back on those years of the early 1960s, the... 

Rapid phosphorylation of the other detected phosphoproteins does occur but no definite roles have yet been ascribed to them. The 33 kDa protein may be the S6 ribosomal protein involved in the control of protein synthesis. The 57 kDa protein has been identified as the regulatory suhunit of the cyclic AMP-dependent protein kinase [44]. Of the other proteins the 76, 43 and 20 kDa may be connected with the microfilaments (43 kDa actin, 76 kDa myosin light chain kinase and 20 kDa myosin light chain) but this must be further investigated. These proteins may only play a permissive role in. steroidogenesis. The fact that the pattern of protein phosphorylation is very similar after stimulation of protein kinase C with phorbol esters supports this because the latter only marginally increase steroidogenesis [18]. 

Incubate the reticulocytes with [3H]leucine, which will be incorporated into proteins. Prepare electron microscope autoradiographs and count silver grains per cell and the number of polysomes. The latter appear as rosettes of five ribosomes in these cells. A statistical comparison between the number of polysomes and the amount of protein synthesized during the incubation time (proportional to the number of silver grains) indicates whether there are nonactive polysomes. In fact, many of the polysomes are inactive i.e., they are switched off (see Chap. 17 for the control of protein synthesis). 

In prokaryotes such as E. coli, most of the control of protein synthesis occurs at the level of transcription. (Refer to Section 18.3 for a discussion of the principles of prokaryotic transcriptional control.) This circumstance makes sense for several reasons. First, transcription and translation are directly coupled that is, translation is initiated shortly after transcription begins (Figure 19.8). Second, the lifetime of prokaryotic mRNA is usually relatively short. With half-lives of between 1 and 3 minutes, the types of mRNA produced in a cell can be quickly altered as environmental conditions change. Most mRNA molecules in E. coli are degraded by two exonucleases, referred to as RNase II and polynucleotide phosphorylase. 

F. Evidence for the Control of Protein Synthesis at the Level of mRNA... 

Role of Phosphorylation-Dephosphorylation Cycles in the Control of Protein Synthesis in Eukaryotes Severo Ochoa, Cesar de Haro, John Siekierka, and Haim Grosfeld... 

In this volume the regulation of metabolism is primarily regarded in terms of the control of protein synthesis. Hence, there is an emphasis on enzyme repression and induction, together with complementary regulatory processes. 

Tobacco mosaic virus is made of RNA and a protein coat. The amino acid sequence of the protein coat is dictated by the base sequence of the tobacco mosaic RNA (to determine the composition of the coat protein). Obviously, much can be learned about the control of protein synthesis by changing the composition of the template RNA. This is precisely what happens after chemical mutagens are applied. Three different mutagens have been used nitrous acid, hydroxyla-mine, and 5-fluorouracil. Among the three, the results obtained with nitrous acid are particularly interesting. 

The control of protein synthesis, either by regulation of the amount of mRNA available for translation or by the efficiency with which it is translated, is important in cell growth and development as a factor determining the level of cellular and extracellular proteins. Subversion of this control occurs in cells infected by viruses when the viral... 

One of the most useful techniques which may be employed in a study of the control of protein synthesis is the use of specific inhibitors, compounds which permit the dissection of the overall sequence into a number of stages by interfering with the biosynthetic process at specific points. A classical inhibitor of protein synthesis in bacteria is chloramphenicol (or Chloromycetin, 97)a broad spectrum antibiotic produced by Streptomyces venezuelae and a concentration of 10-20 micrograms per millilitre will generally decrease the level of protein synthesis by a factor of ten or more. Chloramphenicol may be produced synthetically but of the four stereoisomers only the natural D-threo form (97) shows any significant activity as an inhibitor of protein synthesis in bacteria. 

Wang X, Proud CG. (2006) The mTOR pathway in the control of protein synthesis. Physiology (Bethesda) 21, 362-369. 

Ballinger, D. G., and Pardue, M. L., 1983, The control of protein synthesis during heat shock in Drosophila cells involves altered polypeptide chain elongation rates. Cell 33 103. 

Legon, S., Brayley, A., Hunt, T., and Jackson, R. J., 1974, The effect of cyclic AMP and related compounds on the control of protein synthesis in reticulocyte lysates, Biochem. Biophys. Res. Commun. 56 745. 

From the discussion of the timing of the various groups of prereplicative phage proteins and the transition from prereplicative to late transcription, it would appear that most of the control of protein synthesis after T4 infection occurs on the transcriptional level. However, translational controls have been implicated in the shut-off of host protein synthesis and in the shut-off of early" protein synthesis late in the infectious cycle. 

Levinthal, C., Hosoda, J., Shub, D. The control of protein synthesis after phage infection. In J. S. Colter and W. Paranchych (eds.). The molecular biology of viruses, p. 71-87- New York Academic Press 1967. 

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