Chain termination eukaryotic

Polypeptide chain termination and stop codon readthrough on eukaryotic ribosomes... 

The eukaryotic RNA polymerases are not inhibited by rifamycin, but RNA polymerases II and III are completely inhibited by the mushroom poison a-amanitin (see Box 28-B). Inhibitors of DNA gyrase (Chapter 27) also interfere with transcription as do chain terminators such as cordycepin (3 -deoxyadenosine) and related nucleosides. 

Acts as analog of aminoacyl-tRNA and causes premature chain termination in both prokaryotes and eukaryotes. 

Puromycin causes premature chain termination in both pro- and eukaryotes chloramphenicol binds to 50 S ribosomal subunit and inhibits peptidyl-synthetase streptomycin binds to 30 S ribosome, causing misreading of mRNA, and so on. See Table 12.3. 

Puromycin Causes premature chain termination by acting as an analog of aminoacyl-tRNA (prokaryotes and eukaryotes)... 

The protein synthesis inhibitors tetracycline, chloramphenicol, and streptomycin all block bacterial protein synthesis. Several eukaryotic translational inhibitors have also been found and they include diphtheria toxin, ricin, and cycloheximide. Puromycin causes premature chain termination in both prokaryotes and eukaryotes by functioning as an aminoacyl tRNA analog. 

The eukaryotic FASs synthesize predominantly the 16-carbon saturated product with smaller amounts of 14- and 18-carbon products. The enzymatic basis of product specificity of the animal FAS has been studied in some detail in G. Hammes and S. Smith s laboratories. The KS has a relatively broad chain-length specificity and is able to transfer efficiently saturated acyl moieties with 2-14 carbon atoms from the phosphopantetheine thiol to the active-site cysteine thiol. However, longer chain-length acyl moieties are transferred between thiols with increasing difficulty [17], In contrast, the chain-terminating thioesterase has very limited ability to remove acyl moieties with less than 16 carbon atoms from the phosphopantetheine thiol (S. Smith, 1978, 1991). Thus the specificities of... 

Elongation and termination - Eukaryotic chain termination, in contrast to prokaryotic termination, requires only one protein factor- eRF (Table 28.7), which can recognize all three stop codons (UAA, UAG, and UGA). Otherwise the mechanisms are very similar. 

The details of the chain of events in translation differ somewhat in prokaryotes and eukaryotes. Like DNA and RNA synthesis, this process has been more thoroughly studied in prokaryotes. We shall use Escherichia coli as our principal example, because aU aspects of protein synthesis have been most extensively studied in this bacterium. As was the case with replication and transcription, translation can be divided into stages—chain initiation, chain elongation, and chain termination. 

Eukaryotic chain elongation is similar to the prokaryotic counterpart. With chain termination, there is only one release factor that binds to all three stop codons. 

The m-RNA transcribed from DNA by RNA polymerase has a sequence which contains the information necessary for determining the whole amino acid sequence of the protein. Since a great many different proteins need to be synthesised, there are many thousands of different m-RNA molecules which have to be synthesised in the cell. Most chains of eukaryotic m-RNA are terminated at the 5 end by a cap of 7-methylguanosine which is linked to the chain by a triphosphate group. At the 3 end there is usually a poly A tail consisting of about 100 successive nucleotides with adenine (A) bases (11.129). 

The fundamental process is identical in prokaryotes and eukaryotes, in that an RNA polymerase complex binds to the promoter and initiates transcription at a start site downstream to the promoter. De novo initiation of an RNA chain occurs with a purine nucleotide and creation of a transcription bubble with the open complex. The transcription complex can slide back along the nascent chain and en-donucleolytically cleave off the 3 segment, then moves forward along the DNA template chain termination occurs at specific regions in the genes. 

A bacterial phosphatidylinositol specific phospholipase C (PI-PLC) had been available for many years before it was demonstrated to strip a number of membrane-bound proteins from eukaryotic cell surfaces [1], Such proteins are anchored by a PI moiety in which the 6 position of inositol is glycosidically linked to glucosamine, which in turn is bonded to a polymannan backbone (Fig. 3-10). The polysaccharide chain is joined to the carboxyl terminal of the anchored protein via amide linkage to ethanolamine phosphate. The presence of a free NH2 group in the glucosamine residue makes the structure labile to nitrous acid. Bacterial PI-PLC hydrolyzes the bond between DAG and phosphati-dylinositols, releasing the water-soluble protein polysac charide-inositol phosphate moiety. These proteins are tethered by glycosylphosphatidylinositol (GPI) anchors. 

Another factor that characterizes molybdenum and tungsten enzymes is that instead of using the metal itself, directly coordinated to amino acid side-chains of the protein, an unusual pterin cofactor, Moco, is involved in both molybdenum- and tungsten-containing enzymes. The cofactor (pyranopterin-dithiolate) coordinates the metal ion via a dithiolate side-chain (Figure 17.2). In eukaryotes, the pterin side-chain has a terminal phosphate group, whereas in prokaryotes, the cofactor (R in Figure 17.2) is often a dinucleotide. 

Puromycin inhibits both prokaryotic and eukaryotic translation by binding to the A site. Peptidyl transferase attaches the peptide to puromycin, and the peptide with puromycin attached at the C-terminus is released, prematurely terminating chain growth. 

Abbreviations aa-tRNA Amino-acyl tRNA eLF Eukaryotic translation initiation factor IF Prokaryotic translation initiation factor eEF Eukaryotic translation elongation factor EF Prokaryotic translation elongation factor eRF Eukaryotic translation termination factor (release factor) RF Prokaryotic translation release factor RRF Ribosome recycling factor Rps Protein of the prokaryotic small ribosomal subunit Rpl Protein of the eukaryotic large ribosomal subunit S Protein of the prokaryotic small ribosomal subunit L Protein of the prokaryotic large ribosomal subunit PTC Peptidyl transferase center RNC Ribosome-nascent chain-mRNA complex ram Ribosomal ambiguity mutation RAC Ribosome-associated complex NMD Nonsense-mediated mRNA decay... 

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