Protein synthesis polysomes

According to the accepted scheme of protein synthesis polysomes consist of ribosomes, mRNA, tRNAs, and several protein factors reversibly combined with ribosomes. Thus the only nonribosomal high molecular weight RNA of polysomes presumably is mRNA. Although it has not yet been excluded that some other types of high molecular weight RNA may be present in polysomes, it seems to be unlikely. 

The rapid repression of pre-existing protein synthesis caused by anaerobic treatment is correlated with a near complete dissociation of polysomes in primary roots of soybeans (Lin Key, 1967) and maize (E.S. Dennis and A.J. Pryor, personal communication). This does not result from degradation of aerobic mRNAs, because the mRNAs encoding the pre-existing proteins remain translatable in an in vitro system at least five hours after anaerobic treatment is initiated (Sachs et al., 1980). This is in agreement... 

What could be the signal for the induction of the cold shock proteins It has been observed that shifting E. coli cells from 37 to 5 °C results in an accumulation of 70S monosomes with a concomitant decrease in the number of polysomes [129]. Further, it has been shown that a cold shock response is induced when ribosomal function is inhibited, e.g. by cold-sensitive ribosomal mutations [121] or by certain antibiotics such as chloramphenicol [94]. These data indicate that the physiological signal for the induction of the cold shock response is inhibition of translation caused by the abrupt shift to lower temperature. Then, the cold shock proteins RbfA, CsdA and IF2 associate with the 70S ribosomes to convert the cold-sensitive nontranslatable ribosomes into cold-resistant translatable ribosomes. This in turn results in an increase in cellular protein synthesis and growth of the cells. 

A ribosome is a cytoplasmic nucleoprotein stmcture that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene. In active protein synthesis, many ribosomes are associated with an mRNA molecule in an assembly called the polysome. 

Zalfa, F., Achsel, T., and Bagni, C. (2006). mRNPs, polysomes, or granules FMRP in neuronal protein synthesis. Curr. Opin. Neurobiol. 16, 265-269. 

Ribosomes (79-87) are small organelles 17-23 nm in diameter. They can exist in clusters known as polysomes or be attached to the er where they bind to pores in the er membrane. A major constituent of the er pore is translocon, the heterotrimetric Sec 61 protein complex. Sec 61 binds to the 80s ribosomes (86). Ribosomes consist of subunits, a 30s subunit (16srRNA and 21 proteins), and a 50s subunit (23s and 5s RNAs, > proteins and the catalytic site of peptidyl transferase). Ribosomes are the sites of protein synthesis. 

In cells that are carrying out intensive protein synthesis, ribosomes are often found in a linear arrangement like a string of pearls these are known as polysomes. This arrangement arises because several ribosomes are translat-... 

Inhibits protein synthesis by binding to the 30S subunit of ribosomes, which blocks formation of the initiation complex, causing misreading of the code on the mRNA template and disrupting polysomes. 

Ribosomes are small granular bodies scattered throughout the cytoplasm, and this is the place where protein synthesis starts. rRNA itself does not directly govern protein synthesis. A number of ribosomes get attached to a chain of mRNA and form a polysome, along which, with mRNA acting as the template, protein synthesis occurs. One of the major functions of rRNA is to bind the ribosomes to the mRNA chain. 

Inside the cell, aminoglycosides bind to specific 30S-subunit ribosomal proteins (S12 in the case of streptomycin). Protein synthesis is inhibited by aminoglycosides in at least three ways (Figure 45-3) (1) interference with the initiation complex of peptide formation (2) misreading of mRNA, which causes incorporation of incorrect amino acids into the peptide and results in a nonfunctional or toxic protein and (3) breakup of polysomes into nonfunctional monosomes. These activities occur more or less simultaneously, and the overall effect is irreversible and lethal for the cell. 

Protein synthesis can be carried out by ribosomes free in the cytosol. In eukaryotes, ribosomes also carry out protein synthesis while bound to the surface of the endoplasmic reticulum. In addition, a given mRNA molecule usually has more than one active ribosome translating it into protein an assembly of several ribosomes on a single mRNA is called a polyribosome, or polysome for short. 

Rapid Translation of a Single Message by Polysomes Large clusters of 10 to 100 ribosomes that are very active in protein synthesis can be isolated from both eukaryotic and bacterial cells. Electron micrographs show a liber between adjacent ribosomes in the cluster, which is called a polysome (Fig. 27-27). The connecting strand... 

Polyribosome (polysome). A complex of an mRNA and two or more ribosomes actively engaged in protein synthesis. 

Narciclasine (215) is an antitumor agent which exerts an antimitotic effect during metaphase by immediately terminating protein synthesis in eukaryotic cells at the step of peptide bond formation (97,101,141,142), apparently by interaction with the ansiomycin area of the ribosomal peptidyl transferase center (142). The alkaloid has also been found to inhibit HeLa cell growth and to stabilize HeLa cell polysomes in vivo (97). Although DNA synthesis was retarded by narciclasine, RNA synthesis was practically unaffected (97,142). Sev-... 

Translating ribosomes in eukaryotes are located in different places in the cell depending on the fate of their proteins. Free polysomes are in the cytoplasm and synthesize cytoplasmic proteins and those that are bound for most intracellular organelles, for example, the nucleus. Members of the second class of polysomes, membrane-bound polysomes, are attached to the endoplasmic reticulum (forming the rough ER), and synthesize exported proteins. In cells that are actively secreting enzymes or hormones (for example, those in the pancreas), most of the protein synthesis occurs on the rough ER. 

As well as being essential for certain reactions Na+ and, to a lesser extent, K+ are important in maintaining the osmotic balance in the cell. The sodium ion concentration is maintained about isotonic largely by the NaCl in the BSS but in Earle s BSS this is supplemented with NaHC03 and to a lesser extent with other salts (see Appendix 1). If the Na+ concentration is increased from the normal 120 mM up to 220 mM it leads to a dissociation of polysomes with concomitant inhibition of protein synthesis and cell growth (Fig. 5.1) (Saborio et al., 1974). 

Cell-free systems capable of synthesising polypeptides have been prepared from protoscoleces of E. granulosus (7), larval T. crassiceps (588) and H. diminuta (633). In general, these studies have demonstrated that protein synthesis in cestodes, although showing some specificity, is similar to that in mammals in that it requires polysomes, amino acid adenylates, aminoacyl-tRNAs, pH 5 fraction, ATP, GTP, magnesium and either sodium or potassium ions. 

Sellinger, O., Azcurra, J. (1970). The breakdown of polysomes and the stimulation of protein synthesis in cerebral mechanisms of defense against seizures. In A. Lajtha (Ed.), Protein metabolism of the nervous system (pp. 519-532). New York Plenum. 

Ribosomes are the site of protein synthesis and exist (1) in the cytoplasm as rosette-shaped groups called polysomes (in immature red blood cells there are usually five per group) (2) on the outer face of the RER or (3) in the mitochondrial matrix, although this last type is different in size and shape from ribosomes in the cytoplasm. Ribosomes are composed of RNA and protein and range in size from 15 to 20 nm. Their central role in protein synthesis is described in Chap. 16. 

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). 

Cell swelling first manifests as dilation of the endoplasmic reticulum (ER). Detachment of ribosomes from the rough ER and dissociation of polysomes into monosomes occurs next because of diminished ATP generation, causing reduced protein synthesis. If hypoxia persists, further membrane dysfunction, manifested by more severe increase in permeability, occurs. Cell surface blebs and loss of microvilli in cells so endowed are ultrastructural correlates of more severe membrane damage (Figure 16.4A,B). The point of irreversibility has not yet been reached if oxygen is restored. Indeed, it is very difficult to define precisely when the cell is irreversibly injured. 

Vectorial translation [31,32]. Polypeptides are made on membrane-bound polysomes. Many of these proteins are synthesized with a 16-30 amino acid extension at the NH2-terminus. This signal sequence is hydrophobic in nature. Protein synthesis and translocation, into or across the membrane, are obligatorily linked. Therefore, the transmembrane movement is co-translational and it is coupled to the elongation of the polypeptide chain. Consequently, the completed polypeptide chain is never present in the compartment where it is synthesized. The polypeptides that do not yet cross the membrane are shorter than the mature protein. Addition of inhibitors of protein synthesis immediately arrest movement of the polypeptide across the membrane. 

Vectorial processing [4,33,34]. The polypeptides are usually made on free polysomes. The transport of the polypeptide chain across the membrane is independent of protein synthesis. In most of the cases the protein is synthesized as a larger precursor, and the completed polypeptide chain is present and even might be accumulated in the compartment where it is synthesized. Addition of protein synthesis inhibitors will not prevent the transport of the completed chains across the membrane. During or immediately after transport across the membrane, chemical... 

The rate of protein synthesis is about 6 peptide bonds per minute, thus, it takes, about 1 to 2 minutes to synthesize an average sized protein. Because mRNA is often several thousand nucleotides in length, the same mRNA molecules can be simultaneously bound by many ribosomes. An mRNA that is bound by multiple ribosomes is called a polysome. Polysomes provide a mechanism for many copies of a protein to be translated from a single mRNA. Polysomes in the cytosol synthesize most of the proteins and enzymes required by the body for intracellular processes such as metabolism. 

Azahypoxanthine prevented cloning of human malignant HeLa S3 cells, but not of normal human cells, whereas 8-azaadenine prevented cloning of either type. °° 8-Azainosine (1 fiM) inhibited colony formation by human epidermoid (type 2) carcinoma cells in culture. In cell-free extracts of HeLa cells, protein synthesis was inhibited by the incorporation of 8-aza-guanine into the mRNA of polysomes, in which it blocked formation of peptide bonds. ... 

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