Ribosome polyribosomal

The single binding site for SM on the 30S ribosomal subunit is the 23S core particle of that subunit. A variety of reactions can follow from this event. One is the induction of conformational change in the complete 70S ribosome. Polyribosomes may break down, and the resulting monosomes could reassociate with mRNA, resulting in abnormal initiation complexes, thus inhibiting normal synthesis. 

Fig. 19. Evidence for long-lived mRNA in seedlings of the cotton plant The seedlings were incubated for 12 hours in (control) or + actinomycin. The ribosomes and polyribosomes were then isolated and separated in a sucrose density gradient. The high peak of the curve shown represents the monomeric ribosomes and to the left of it are the associations of the oligo- to polyribosomes. The ribosome-polyribosome profile after actinomycin treatment is not drastically different from that of the control, allhough RNA synthesis is drastically inhibited (red curves) (modified after Dure and Waters 1965).

Many ribosomes can translate the same mRNA molecule simultaneously. Because of their relatively large size, the ribosome particles cannot attach to an mRNA any closer than 35 nucleotides apart. Multiple ribosomes on the same mRNA molecule form a polyribosome, or polysome. In an unrestricted system, the number of ribosomes attached to an mRNA (and thus the size of polyribosomes) correlates positively with the length of the mRNA molecule. The mass of the mRNA molecule is, of course, quite small compared with the mass of even a single ribosome. 

The field of translation initiation has focused on the initial round ofribosomal subunit recruitment to an mRNA. Presumably, these events are mirrored in the subsequent rounds of initiation necessary for polyribosome formation. Importantly, because mRNAs are typically present in large polyribosomes (averaging 9-13 ribosomes per mRNA), the initiation events that govern ribosome recruitment to preexisting polyribosomes constitute the majority of translation initiation cycles occurring in an mRNA s lifetime. Whether or not these initiation events mimic the first round of initiation is not yet known. Since eukaryotic cells divide ribosomes between two subcellular compartments, the cytosol and ER membrane, it is also important to know if the mechanism of translation initiation on ER-bound ribosomes is similar to that occurring on soluble ribosomes and, importantly, whether ER-bound ribosomes can direcdy (re) initiate translation on bound polyribosmes. 

The electron microscope (EM) has been used extensively in molecular biological research (1). Among its many applications, EM has been used to visualize polyribosomal structure (2), to visualize ribosome substructure (3), and to visualize the elongation factor Tu on the Escherichia... 

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.

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. 

Translation begins at the 5 -end of the mRNA, with the ribosome proceeding along the RNA molecule. Because of the length of most mRNAs, more than one ribosome at a time can generally translate a message (Figure 31.14). Such a complex of one mRNA and a number of ribosomes is called a polysome or polyribosome. 

A polyribosome consists of several ribosomes simultaneously translating one mRNA. 

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... 

Polyribosomes. Under suitable conditions ribosomes isolated from cells are found to sediment together in clusters, often of five or more. These polyribosomes (or polysomes), which can be seen in electron micrographs (Fig. 28-5), are held together by chains of mRNA. Polyribosomes arise because a single mRNA molecule is being translated by several ribosomes at once. As the 5 terminus of the mRNA emerges from one ribosome, it may soon combine with another and initiate translation of a second peptide chain, etc. The length of the mRNA determines how many ribosomes are likely to be associated in a polyribosome. 

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

In addition, several ribosomes can independently and simultaneously translate a mRNA molecule and, hence, synthesize several identical polypeptide chains concurrently (Figure 12.3). Such clusters or groups of ribosomes are called polyribosomes or polysomes. The number of attached ribosomes depends on the size of the mRNA and how frequently ribosomes can initiate at the start of a gene sequence. Because RNA transcription and translation are neither temporally nor spatially separated in prokaryotes, it is possible for translation to begin before transcription is completed. However, we have already noted that prokaryotic mRNAs have short half-lives this is probably a result of their continuous degra-... 

Yes. It is common for any single mRNA to be translated simultaneously by many ribosomes. They give rise to a structure called a polyribosome or polysome. 

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. 

Pestka, S., Studies on transfer ribonucleic acid-ribosome complexes. Effect of antibiotics on peptidyl puromycin synthesis on polyribosomes from Escherichia coU. J. Biol. Chem. 1972, 247, 4669-4678. 

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 . 

The linear deoxyribonucleotide sequence information encoded in genes is copied into a linear sequence of ribonncleotides in messenger RNA (mRNA) by the process of transcription. Ribosomes bind one at a time near to the 5 end of each mRNA molecule and translate the ribonucleotide base sequence into an aminoacyl sequence according to the genetic code. Each mRNA is bound to more than one ribosome at a time, forming a polyribosome, often abbreviated to polysome. Typically the rate of... 

Sparsomycin, a sulfur-containing antibiotic, inhibits protein synthesis in mammalian and bacterial cells. Tryptophan administration before or after sparsomycin did not affect the hepatic polyribosomal disaggregation or the decreased protein synthesis due to sparsomycin.188 A possible explanation for the lack of effect by tryptophan may be due to sparsomycin s ability to cause fall-off ribosomes, which are defective as indicated by the decreased formation of polyphenylalanine when assayed in vitro with poly(U).207... 

Sarma, D. S., Reid, I. M., Verney, E., and Sidransky, H., Studies on the nature of attachment of ribosomes to membranes in liver. I. Influence of ethionine, sparsomycin, CC14, and puromycin on membrane-bound polyribosomal disaggregation and on detachment of membrane-bound ribosomes from membranes, Lab. Invest., T7, 39, 1972. 

Fig. 3-14 Electron micrograph showing polyribosomes from the bacterium Escherichia coli. Filaments are DNA molecules. Ribosomes are attached to mRNA which they are translating. Bar = 0.1 /rm. Photograph courtesy of Dr. O. L. Miller, Jr. (from Miller ef a/., 1970).

---