Steps in Translation

Covalent coupling of amino acids to their tRNAs is a high-fidelity process mediated by very specific enzymes. 

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Aminoacjrl-tRNA synthetases are the critically important enzymes responsible 

Proteins are costly to the cell, requiring hydrolysis of five high-energy phosphate bonds per amino acid incorporated. 

Ribosomes are the ribonucleoprotein machines that translate the mRNA into 

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Viral IRES elements can be useful tools to identify the sub-step in translation targeted by a given regulator (Jackson, 2005 Ostareck et al., 2001). The IRES elements listed in Table 6.1 are of particular interest, because careful biochemical and structural analyses have defined different initiation factor requirements for translation in each case (Borman and Kean, 1997 Fraser and Doudna, 2006 Hellen and Sarnow, 2001 Poyry et al, 2004). We and others have employed viral IRES-containing reporter constructs to... 

C. The first step in translation of an mRNA is assembly of an initiation complex of... 

The Code Is Not Quite Universal The Rules Regarding Codon-Anticodon Pairing Are Species-Specific The Steps in Translation... 

The last step in translation involves the cleavage of the ester bond that joins the now complete peptide chain to the tRNA corresponding to its C-terminal amino acid (fig. 29.19). Termination requires a termination codon, mRNA and at least one protein release factor (RF). The freeing of the ribosome from mRNA during this step requires the participation of a protein called ribosome releasing factor (RRF). 

Time out for units Now we can consider more carefully the incremental change in the response of a medium to which small particles are added in dilute suspension. Units can be a nuisance, so put the facts first. Focus on the physically important change in polarization 3P/3 N with the addition of one particle. Then it is worthwhile to work slowly through the steps in translation. 

To remove the now uncharged fMet-tRNA from the P site, the ribosomal complex will shift toward the 3 end of the mRNA by three bases, positioning a new codon in the A site and ejecting the spent fMet-tRNA. At this point, the dipeptide bound by the tRNA that formerly occupied the A site now occupies the P site. This final step in translation elongation is fueled by the hydrolysis of another GTP molecule by a translocase enzyme called elongation factor G (EF-G, Fig. 23-5). 

Another difference between prokaryotic and eukaryotic translation is the nucleic acid sequences that recruit the small ribosomal subunit to the mRNA. As stated earlier, the Shine-Delgarno sequence interacts with the 16S rRNA of the 3 OS ribosomal subunit in the prokaryotic system, the critical step in translation initiation. In the eukaryotic... 

Match each antibiotic below with the appropriate step in translation that it inhibits in prokaryotes. 

Mammals are attractive models to study ototoxicity, since the anatomical characteristics of the inner ear and drug-dependent inner ear pathologies are similar to humans. Experimental designs generally include the measurement of auditory potentials (mostly nonin-vasive ABRs) and postmortem evaluation of cochlear pathology. Overall, mammalian models are an essential step in translational research. 

The first step in translation is the assembly of the initiation complex. In bacteria, this is done in a three-step process that involves the initiation factors IF-1, IF-2, and IF-3, the 30S and 50S ribosomal subunits, and the hydrolysis of GTP. As shown in Figure 26.10, the first step is the formation of a preinitiation complex consisting of IF-1, IF-3, and the 30S ribosomal subunit. This preinitiation complex binds with high affinity to a translational start site on the mRNA transcript which is close to the 50 end and includes the first codon of the open-reading frame. This codon is called the initiation codon and it is usually AUG, which specifies the amino-acid methionine in the genetic code (Fig. 26.6). 

Therefore, enzyme-catalyzed reactions in which irreversibility is important take place by one of the mechanisms that form pyrophosphate as a product (attack on the a- or i8-phosphorus of ATP). For example, both the reaction that links nucleotide subunits to form nucleic acids (Section 27.7) and the reaction that binds an amino acid to a tRNA (the first step in translating RNA into a protein Section 27.12) involve nucleophilic attack on the a-phosphoms of ATP. 

Although the RNA world presumably existed almost four billion years ago, X-ray-structures of the ribosome seem to exhibit fragments of this ancestral era. The structures revealed that RNA-mediated catalysis plays an important role in the peptide synthesis of the ribosome. The key step in translation is catalysed only by the ribonucleic acid component of the ribosome, without any direct contribution of proteins from the spatial vicinity. That impressively demonstrates the catalytic potential of RNA in a biochemical reaction that may arguably be called the most important ever. 

Although the synthesis of a peptide bond is the key step in translation, this is the easiest part of protein synthesis. Once the amino group of an aminoacyl-tRNA is properly positioned close enough to the carbonyl group of a peptidyl-tRNA, peptide bond formation through nucleophilic attack is energetically favorable. The ribosome can be considered as a single enzyme whose function is to catalyze peptide bond formation. 

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