Elongation phase of protein synthesis

Elongation factors. Protein factors uniquely required during the elongation phase of protein synthesis. Elongation factor G (EF-G) brings about the movement of the peptidyl-tRNA from the A site to the P site of the ribosome. 

When the 70S initiation complex has been formed, the ribosome is ready for the elongation phase of protein synthesis. The fMet-tRNAf molecule occupies the P site on the ribosome. The other two sites for tRNA molecules, the A site and the E site, are empty. Formylmethionyl-tRNAf is positioned so that its anticodon pairs with the initiating AUG (or GUG) codon on mRNA. This interaction sets the reading frame for the translation of the entire mRNA. 

Elongation phase of protein synthesis binding of charged tRNA, peptide bond formation, and translocation. Note that this phase consumes 2 GTP during each cycle of chain elongation. 

Peptidyl transferase reaction in the elongation phase of protein synthesis. 

During translation, the ribosome binds to the mRNA at the start codon (A UG) that is recognized only by the initiator tRNA. The ribosome proceeds to the elongation phase of protein synthesis. During this stage, complexes, composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into poly-peptidic sequences dictated by DNA and represented by mRNA. 

The extent of methylation of a gene is correlated with its ability to transcribe. Given that DNA methylation usually reduces transcription, two important, closely related questions remain unanswered How is methylation regulated in vivo How does methylation interfere with transcription Since methylation is known not to interfere with the elongation phase of RNA synthesis, it seems likely that methylation blocks initiation. The binding of polymerase and other regulatory proteins at the initiation locus is sensitive to modification of these nucleotides. The precise inhibition mechanisms, however, await further elucidation. 

The second phase of protein synthesis is the elongation cycle. This phase begins with the insertion of an aminoacyl-lRKA into the empty A site on the ribosome. The particular species inserted depends on the mRNA codon in the A site. The cognate ami noacyl-tKN A does not simply leave the synthetase and diffuse to the A site. Rather, it is delivered to the A site in association with a 4. Tkd protein called elongation factor Tu (EF-Tu). Elongation factor Tu, another member of the G-protein family, requires GTP to bind aminoacyl-tRNA (Figure 30.23) and to bind the ribosome. The binding of... 

The large (50s) subunit contains 23s rRNA, 5s rRNA, and over 30 proteins, and the small (30s) subunit contains 16s rRNA and about 20 proteins. The complete 70s ribosome is formed by association of the 30s and 50s subunits through a networks of bridges. The 30s subunit mediates the interaction between mRNA codons and tRNA anticodons on which the fidelity of protein translation depends. The 50s subimit includes the activity that catalyzes peptide bond formation—peptide transferase and the binding site for GTP— which are the binding protein factors that assist in the initiative elongation and termination phases of protein synthesis. 

Inhibition of protein synthesis in eukaryotic cells by the Cephalotaxus alkaloids harringtonine, homoharringtonine, and isoharringtonine has been studied.16 In model systems, these alkaloids were found not to inhibit any of the initiation steps but to block certain parts of the elongation phase of translation. 

The elongation phase of transcription is affected by the presence or absence of specific protein factors, called elongation factors, such as GreA and GreB for E. coli RNA Pol and TFllS for eukaryotic RNA Pol 11. The stimulation of transcriptional elongation is attributed, at least in part, to the elongation factor-associated 3 — 5 hydrolytic activity which enables the stalled RNA Pol complex to retract from the transcription barriers (e.g., pause sites) and to repeat the forward synthesis, eventually overcoming the barriers. 

Protein synthesis takes place in three phases initiation, elongation, and termination. In prokaryotes, mRNA, formylmethionyl-tRNAf (the special initiator tRNA that recognizes AUG), and a 308 ribosomal suhunit come together with the assistance of initiation factors to form a 308 initiation complex. A 508 rihosomal suhunit then joins this complex to form a 708 initiation complex, in which fMet-tRNAf occupies the P site of the rihosome. 

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