Polyribosomes protein synthesis

Malkin, L. I., Gross, P. R., and Romanoff, P. (1964). Polyribosomal protein synthesis in fertilized sea urchin eggs The effect of actinomycin treatment. Develop. Biol. 10, 378-394. 

Bewley, J.D. Larsen, K. (1980). Cessation of protein synthesis in water-stressed pea roots and maize mesocotyls without loss of polyribosomes. Effects of lethal and non-lethal water stress. Journal of Experimental Botany, 32, 1245-56. 

Drought also has a profound effect on protein synthesis. In many plant tissues, a reduced water potential causes a reduction of total protein synthesis and a rapid dissociation of polyribosomes. The latter has been shown not to be the consequence of increase in ribonuclease activity (Hsiao, 1973 Dhindsa Bewley, 1976). For a specific protein, Jacobsen, Hanson Chandler (1986) have shown in barley leaves that water stress enhances the synthesis of one of the a-amylase isozymes. Using a cDNA probe they found that water-stressed leaves contained much more a-amylase mRNA than unstressed plants. 

Lin, C.Y. Key, J.L. (1967). Dissociation and reassembly of polyribosomes in relation to protein synthesis in the soybean root. Journal of Molecular Biology, 26, 237-47. 

Figure 1.8 Translation of messenger RNA. The attachment of a ribosome to the mRNA involves protein initiation factors and the recognition of a particular base sequence, the start codon. A single mRNA can be simultaneously translated by more than one ribosome, forming a polyribosome. Synthesis occurs in the direction from the 5 end of messenger RNA to the 3 end. For further details of protein synthesis see Chapter 20.

The relative importance of hepatic microsomal lipid peroxidation versus covalent binding of carbon tetrachloride-derived radicals has been the subject of considerable debate. Since cytochrome P-450 loss has been shown to be related to lipid peroxidation and to covalent binding, each in the absence of the other, both of these early consequences of carbon tetrachloride metabolism may contribute to P-450 destruction. Nevertheless, it is still not clear how these initial events are related to subsequent triglyceride accumulation, polyribosomal disaggregation, depression of protein synthesis, cell membrane breakdown and eventual death of the hepatocytes. Carbon tetrachloride... 

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. 

In bacteria transcription and translation are closely linked. Polyribosomes may assemble on single DNA strands as shown in Fig. 28-5. It has often been assumed that RNA synthesis occurs on loops of DNA that extend out into the cytosol. However, recent studies indicate that most transcription occurs in the dense nucleoid and that assembly of ribosomes takes place in the cytosol.2683 In a similar way eukaryotic transcription occurs in the nucleus and protein synthesis in the cytosol. Nevertheless, some active ribosomes are present in the nucleus.26813... 

Once inside the coil, virulent viruses turn off cellular macromolccular synthesis and disaggregate cellular polyribosomes, thus favoring a shift to viral synthesis. These viruses cause the ultimate destruction of the infected cell. In contrast, moderate viruses may stimulate host DNA, ntRNA, and protein synthesis—a phenomenon which may be of considerable importance in viral carcinogenesis. 

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

Increased production of metabolites of phenylalanine that inhibit synthesis of a variety of substances required for normal brain growth Inhibition of TV-methyl-D-aspartate receptors, which are involved in memory and learning Competitive inhibition of transport of other amino acids required for protein synthesis Impaired polyribosome formation or stabilization Reduced synthesis/increased degradation of myelin Decreased formation of norepinephrine and serotonin Altered myelin structure and function... 

A close look at the events which occur during the lag period In GA Induction of novo synthesis of new enzymes has provided some Important clues as to whether GA acts at the transcriptional or translational level (16 and papers cited therein). An increase in polyribosome formation and an increased synthesis of ribosomes and endoplasmic reticulum membranes were found. All of these effects begin within 2 to 4 hr after application of GA. Their observations led Evins and Varner (16) to conclude that the GA-stimulated increases in the number of monoribosomes and the percentage of polyribosomes probably are prerequisite for the hormone induction of protein synthesis. 

Once protein synthesis is initiated, amino acids are added to the peptide chain corresponding to each triplet in the mRNA until the ribosome encounters a termination or stop codon, whereupon the polypeptide chain is released from the ribosome, and assumes its final configuration. A ribosome covers about 50 bases of an mRNA, which is usually hundreds of bases long. Thus, several ribosomes translate an mRNA consecutively and simultaneously at any instant as shown in Fig. 2.4. A group of ribosomes translating a message is called a polyribosome . 

Early mechanistic studies in the 1960s demonstrated that tylophorine alkaloids irreversibly inhibit DNA synthesis and affected protein synthesis at the elongation stage of the translation cycle [84], In HeLa cells, tylophorine (1) reversibly inhibits RNA synthesis and irreversibly inhibits DNA synthesis, but a predominant effect is exerted on protein synthesis and elongation of peptide ehains by preventing breakdown of polyribosomes and release of nascent peptides. Tyloerebrinc (2) was shown to have equivalent activity to emetine on inhibition of protein synthesis in Entamoeba histolytica [85]. Tylophorine and tylocrebrine were also found to inhibit protein synthesis in Ehrlich ascites cells and... 

Sidransky, H., Sarma, D. S. R., Bongiorno, M., and Verney, E., Effect of dietary tryptophan on hepatic polyribosomes and protein synthesis in fasted mice, /. Biol. Chem., 24, 1123, 1968. 

The laboratory of Sidransky et al.52-53 62 in 1967-1968 demonstrated that L-tryptophan alone elicited a response of hepatic polyribosomes toward heavier aggregation as well as an enhancement of protein synthesis as measured in vivo or in vitro. The single administration of tryptophan alone, but not of one of three other essential amino acids (threonine, methionine, or isoleucine), elicited a stimulatory hepatic response that was similar to that obtained with a complete amino acid mixture.53 Earlier, Feigelson et al.63 in 1959 had reported that the parenteral administration of tryptophan in the rat produced a transient elevation in hepatic protein synthesis as determined by measuring 2[14C]glycine incorporation into hepatic protein. Subsequent studies from other laboratories60 64-69 confirmed the stimulatory effect of tryptophan on hepatic protein synthesis. 

After establishing that L-tryptophan alone stimulated overall hepatic protein synthesis, it became important to determine which proteins were involved. Increased synthesis of albumin due to L-tryptophan was reported by Rothschild et al.65 and by Jorgensen and Majumdar.47/18 The latter investigators also reported that tryptophan increased the synthesis of transferrin, fibrinogen, and ferritin. Thus, extracellular as well as intracellular proteins that are synthesized by the liver were affected by L-tryptophan. Furthermore, it was demonstrated that after L-tryptophan administration, both free and membrane-bound polyribosomes of the liver showed a shift toward heavier aggregation, more marked for free polyribosomes than for membrane-bound polyribosomes.62 Also, in vitro protein synthesis revealed a greater increase with free polyribosomes than with membrane-bound polyribosomes of the livers of tryptophan-treated animals in comparison with similar fractions of livers of control animals. 

Moja et al.57 treated rats with cycloheximide, an inhibitor of protein synthesis, and then fed a tryptophan-free mixture. The cycloheximide treatment largely prevented the marked decrease of plasma tryptophan (total and free). These data supported the hypothesis that protein synthesis was the mechanism through which the ingested tryptophan-free mixture decreased blood tryptophan levels. However, this interpretation was not consistent with the findings of Wunner et al.58 and Sidransky et al.,59 who observed a low rate of polyribosome formation and low incorporation of labeled amino acids in the livers of rodents previously fasted and then acutely fed a tryptophan-free amino acid mixture. 

Many reports years ago dealt with the toxic effect of carbon tetrachloride on the liver of experimental animals.182 183 Among the biochemical changes in the liver following the administration of carbon tetrachloride are disaggregation of hepatic polyribosomes and inhibition of hepatic protein synthesis.184-186... 

Rothschild et al.109 found that the addition of tryptophan to isolated perfused livers of rabbits treated with CC14 aided in the recovery of albumin synthesis. This study is consistent with the findings that tryptophan had a beneficial effect on hepatic polyribosomes and protein synthesis in rats treated with CC14. Thus, tryptophan may be a useful therapeutic agent for the livers of experimental animals injured with CC14. 

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