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 template properties of virus RNA are manifested, as in the case of messenger RNA of the host cell, during its interaction with ribosomes. Just as in the case of normal intracellular synthesis, polysome structures of the type illustrated in Fig. 8 are formed under these circumstances. The formation of active polysomes during interaction between virus RNA and cell ribosomes have been studied in particular detail in the case of poliovirus infections of HeLa cell cultures (Penman et al., 1963 Scharff et al., 1963 Rich et al., 1963). In this condition virus-specific polysomes are formed which are much bigger than the polysomes of uninfected HeLa cells, as a result of the polycistron character of virus RNA. Some of them contained as many as 60 ribosomes. 

KC) EtdBr RNA synthesis Polysome stability Protein synthesis Mitochondrial polypeptide initiation ... 

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. 

The same mRNA molecule can be read many times, so that further initiations can begin before the whole mRNA has been translated. Therefore, several ribosomes (small and large) may be attached to a single mRNA molecule to form a polysome. The number of polysomes in a cell is sometimes used as a rough indication of the extent of peptide synthesis in that cell. 

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... 

Some proteins, especially those destined for the eukaryotic mitochondria and chloroplasts, are transported after their synthesis on free polysomes is complete. Such transport is known as posttranslational transport. In the case of posttranslational transport it is believed that the polypeptide to be transported must be unfolded from its native folded configuration by a system of polypeptide-chain-binding proteins (PCBs) before it can pass through the membrane. Posttranslational transport into the mitochondrion requires both ATP and a proton gradient. Presumably the energy from one or both of these sources is used to unfold the protein or separate it from the PCB system so that it can pass through the membrane. 

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-... 

From a critical study of the metabolism of poly (A) in auxin-treated pea epicotyl, Verma and Maclachlan (73) showed that discreet classes of poly (A) (presumably part of mRNA s) are differentially associated with free and membrane-bound polysomes. The induction of specific mRNA s, the decline in the rate of synthesis of mRNA s, the polysome content per cell, and the formation of cellulase were all related to the membrane-bound polysomes. Although the rate of in vivo enzyme synthesis is... 

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. 

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