Ribosomes initiation

Several key concepts are worth remembering. GTP is used as an energy source for translation, but ATP is used to form the aminoacyl-tRNA. The ribosome effectively has two kinds of tRNA binding sites. Only tRNAMet can bind to the P (for peptide) site, and this only occurs during the initial formation of the functional ribosome (initiation). All other aminoacyl-tRNAs enter at the A (for amino acid) binding site. After formation of the peptide bond (this doesn t require GTP hydrolysis), the tRNA with the growing peptide attached is moved (translocated) to the other site (this does require GTP hydrolysis). 

FIGURE 27-6 Effect of a termination codon in a repeating tetranucleotide. Termination codons (pink) are encountered every fourth codon in three different reading frames (shown in different colors). Dipeptides or tripeptides are synthesized, depending on where the ribosome initially binds. 

Functional 70S ribosome (initiation complex) Aminoacyl-tRNAs specified by codons Elongation factors (EF-Tu, EF-Ts, EF-G)... 

At each stage in protein synthesis on the ribosome—-initiation, elongation, and termination—a different set of protein factors is engaged by the ribosome. Why do such protein factors, which are crucial to the translation, exist separate from the ribosome Why must they cycle on and... 

The context of the mRNA sequence around the AUG codon translational start site, where the ribosome initiates translation of protein synthesis, is an important consideration. An appropriate consensus ribosome-binding region for the host organism should be used and potentially inhibitory RNA secondary structure, that may affect the ability of the ribosome to access this translation start site, avoided. Various RNA structure prediction programs such as... 

For many years, ribosomal proteins were presumed to orchestrate protein synthesis and ribosomal RNAs were presumed to serve primarily as structural scaffolding. The current view is almost the reverse. The discovery of catalytic RNA made biochemists receptive to the possibility that RNA plays a much more active role in ribosomal function. The detailed structures make it clear that the key sites in the ribosome are composed almost entirely of RNA. Contributions from the proteins are minor. Many of the proteins have elongated structures that "snake" their way into the RNA matrix (Figure 29.18). The almost inescapable conclusion is that the ribosome initially consisted only of RNA and that the proteins were added later to fine tune its functional properties. This conclusion has the pleasing consequence of dodging a "chicken and egg" question—namely, How can complex proteins be synthesized if complex proteins are required for protein synthesis ... 

Translation (1RNA ribosomes initiation, elongation, and termination (actors)... 

Chain initiation Initiation requires the smaller ribosomal subunit, an initiation tRNA (with a 5 -CAU-3 anticodon), mRNA with its initiator codon (S -AUG-3 ), and several initiation factors, all of which form the ribosomal initiation complex. The initiation complex is near completion when a tRNA carrying the amino acid methionine hydrogen bonds to the AUG codon on the mRNA. When these components are in place, the larger ribosomal subunit joins the complex in such a way that the initiator met-tRNAjjj j is localized in the P- or peptidyl site (Figure 9-1). The chain initiation phase ends and a second amino acid can be inserted. 

See also Structure of Prokaryotic mRNAs, Structure of tRNAs, Ribosomes, Initiation of Translation, Elongation of Translation, Termination of Translation... 

Heme also regulates the synthesis of hemoglobin by stimulating synthesis of the protein globin. Heme maintains the ribosomal initiation complex for globin synthesis in an active state (see Chapter 15). 

Aminoglycosides are bactericidal inhibitors of protein synthesis (Table 7.4). They bind the 30s or 50s ribosomal subunit and destroy the mRNA-ribosome initiation complex, which is an essential part of protein synthesis (Fig. 7.14). Penicillins and cephalosporins enhance the actions of aminoglycosides by preventing cell wall repair. This results in "holes" in the cell wall that serve as a portal for aminoglycoside entry. 

In addition to mechanisms designed to control the rate at which ribosomes initiate the synthesis of certain specific proteins in animal cells (Sections III,C and D) there is also evidence for a more general regulation of the rate of initiation on all mRNA s, and this will be discussed briefly below. 

Other investigators have studied translation of poliovirus ENA in non-preincubated cell-free systems prepared from infected HeLA cells, in which ribosomes initiate and elongate on endogenous viral mRNAs which were in the cell at the time of extract preparation (32, 33) These extracts synthesize polypeptides which show excellent correlation with those made in infected cells (32, 34, Ehrenfeld, unpublished observations). Neither detectable premature termination nor cleavage deficiencies are apparent. 

Person, A., Ben-Hamida, F., and Beaud, G., 1980, Inhibition of 40 S-Met-tRNAf ribosomal initiation complex formation by vaccinia virus. Nature 287 355. 

Schreier, M. H., and Staehelin, T., 1973, Initiation of eukaryotic protein synthesis (Met-tRNAf.40 S ribosome) initiation complex catalysed by purified initiation factors in the absence of mRNA, Nature New Biol. 242 35. 

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