Translation ribosomes

Finally, to produce the structural and functional devices of the cell, polypeptides are synthesized by ribosomal translation of the mRNA. The supramolecular complex of the E. coli ribosome consists of 52 protein and three RNA molecules. The power of programmed molecular recognition is impressively demonstrated by the fact that aU of the individual 55 ribosomal building blocks spontaneously assemble to form the functional supramolecular complex by means of noncovalent interactions. The ribosome contains two subunits, the 308 subunit, with a molecular weight of about 930 kDa, and the 1590-kDa 50S subunit, forming particles of about 25-nm diameter. The resolution of the well-defined three-dimensional structure of the ribosome and the exact topographical constitution of its components are still under active investigation. Nevertheless, the localization of the multiple enzymatic domains, e.g., the peptidyl transferase, are well known, and thus the fundamental functions of the entire supramolecular machine is understood [24]. 

The ribosome translates the message in the 5 to 3 direction, synthesizing the protein from amino terminus to carboxyl terminus. 

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. 

Most recently, functioning translation systems that included ribosomes have been encapsulated in lipid vesicles. The first attempt to assemble a translation system in a lipid vesicle system was made by Oberholzer et al. [84], However, only very small amounts of peptides were synthesized, largely because the lipid bilayer was impermeable to amino acids, so that ribosomal translation was limited to the small number of amino acids encapsulated within the vesicles. Yu et al. [85] and Nomura et al. [86] improved the yield... 

Any organic compound containing an amino (NH2) and a carboxyl (COOH) group there are 20 a-amino acids from which proteins are synthesized during ribosomal translation of mRNA... 

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 synthesis of proteins involves converting the nucleotide sequence of specific regions of DNA into mRNA (transcription), followed by the formation of peptide bonds in a complex set of reactions that occur on ribosomes (translation). 

The genetic code is a mechanism by which ribosomes translate nucleotide base sequences into the primary sequence of polypeptides. 

Termination. During termination the polypeptide chain is released from the ribosome. Translation terminates because a stop codon cannot bind an aminoacyl-tRNA. Instead, a protein releasing factor binds to the A site. Subsequently, pep-tidyl transferase (acting as an esterase) hydrolyzes the bond connecting the now-completed polypeptide chain and the tRNA in the P site. Translation ends as the ribosome releases the mRNA and dissociates into the large and small subunits. 

Other organelles include the rough endoplasmic reticulum, the subsurface cisternae, and the mitochondria. As in other cell types, the rough endoplasmic reticulum is the primary site of protein synthesis. In neurons, this granular organelle studded with ribosomes is also known as the Nissl substance, due to its chromophilic nature. It is highly developed in neurons and reflects the active protein synthesis that is constantly occurring in these cells. Few of the proteins synthesized at the Nissl bodies are secreted and the majority of them remain within the cell or are incorporated into the cell membrane. After ribosomal translation, many of the proteins are transported... 

The drugs described in Table 7.5, like aminoglycosides, inhibit protein synthesis by interfering with ribosomal translation. These agents differ from aminoglycosides however, in that they are bacteriostatic rather than bactericidal. 

Experiments were then performed to test the validity of the concept of spare receptors as the explanation of the alteration in value during desensitization in NCB-20 cells. It was shown first that desensitization to PGI2 responsiveness was mediated by a relatively rapid loss of membrane receptors k = 5.08 x 10 s ). In other experiments, however, cells were cultured in the presence of cycloheximide (an inhibitor of ribosomal translation that arrests totally new protein synthesis) which resulted in a substantially slower loss of both adenylate cyclase molecules as measured by basal enzyme activity k = 1.94 X 10" s ), and PGI2 receptors (fc = 0.92 x 10 s ). 

In bacterial cells mRNA is combined with ribosomes immediately after its synthesis (Stent, 1966). In this way the ribosomes perhaps have a role in the liberation of the newly formed mRNA from the DNA template. As a result, a transcription-translation complex is formed containing DNA, RNA polymerase, the newly synthesized growing RNA chain, and ribosomes translating mRNA. The transcription and translation are coupled. On the other hand in eukaryotes transcription and translation are separated in... 

Thus the information necessary to specify the structures of thousands of different proteins (and enzymes) in the average living cell is stored in the base sequence of the DNA. This information is transcribed from DNA to m-RNA which carries it to the ribosomes where protein synthesis occurs. At the ribosomes, translation of the codons of the nucleic acid structure into the 20-letter alphabet of amino acids required for protein sequences, is accomplished by t-RNA. 

The photoactivated analog retains the functional activity of streptomycin by two criteria (1) it binds only to the 30 S ribosomal subparticle, and (2) it inhibits the factor-free ( nonenzymic ) p-chloromercuriben-zoate-stimulated poly (U)-dependent ribosomal translation system. ... 

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