Translation Md. Habibur Rahaman (HbR) Dept. of Biochemistry and Microbiology North South University

Stages of Translation Process Protein synthesis can be conveniently divided into four stages: The binding of amino acids to the tRNAs (2) Initiation, in which the components necessary for translation are assembled at the ribosome (3) Elongation, in which amino acids are joined, one at a time, to the growing polypeptide chain (4) Termination, in which protein synthesis halts at the termination codon and the translation components are released from the ribosome.

The Binding of Amino Acids to Transfer RNAs Cell typically possesses from 30 to 50 different tRNAs Collectively, these tRNAs are attached to the 20 different amino acids. Each tRNA is specific for a particular kind of amino acid. All tRNAs have the sequence CCA at the 3’ end, and the carboxyl group (COO ) of the amino acid is attached to the 2’- or 3’-hydroxyl group of the adenine nucleotide at the end of the tRNA

If each tRNA is specific for a particular amino acid but all amino acids are attached to the same nucleotide (A) at the 3’ end of a tRNA, how does a tRNA link up with its appropriate amino acid? The key to specificity between an amino acid and its tRNA is a set of enzymes called aminoacyl-tRNA synthetases. A cell has 20 different aminoacyl-tRNA synthetases, one for each of the 20 amino acids. Each synthetase recognizes a particular amino acid, as well as all the tRNAs that accept that amino acid. Recognition of the appropriate amino acid by a synthetase is based on the different sizes, charges, and R groups of the amino acids.

The tRNAs, however, are all similar in tertiary structure The tRNAs, however, are all similar in tertiary structure. How does a synthetase distinguish among tRNAs? The recognition of tRNAs by a synthetase depends on the differing nucleotide sequences of tRNAs. Researchers have identified which nucleotides are important in recognition by altering different nucleotides in a particular tRNA and determining whether the altered tRNA is still recognized by its synthetase. The results of these studies revealed that the anticodon loop, the DHU-loop, and the acceptor stem are particularly critical for the identification of most tRNAs

The attachment of a tRNA to its appropriate amino acid (termed tRNA charging) requires energy, which is supplied by adenosine triphosphate (ATP): This reaction takes place in two steps: Errors in tRNA charging are rare; they occur in only about 1 in 10,000 to 1 in 100,000 reactions. This fidelity is due to the presence of proofreading activity in the synthetases, which detects and removes incorrectly paired amino acids from the tRNAs.

The Initiation of Translation Prokaryotic Initiation System The second stage in the process of protein synthesis is initiation. During initiation, all the components necessary for protein synthesis assemble: mRNA the small and large subunits of the ribosome a set of three proteins called initiation factors initiator tRNA with N-formylmethionine attached (fMet-tRNAfMet ) guanosine triphosphate (GTP). Initiation comprises three major steps.

The Initiation of Translation Prokaryotic Initiation System Initiation comprises three major steps. First, mRNA binds to the small subunit of the ribosome. Second, initiator tRNA binds to the mRNA through base pairing between the codon and anticodon. Third, the large ribosome joins the initiation complex.

The Initiation of Translation Prokaryotic Initiation System

The Initiation of Translation Prokaryotic Initiation System The sequence covered by the ribosome during initiation is from 30 to 40 nucleotides long and includes the AUG initiation codon. Within the ribosome-binding site is the Shine-Dalgarno consensus sequence, which is complementary to a sequence of nucleotides at the 3’ end of 16S rRNA (part of the small subunit of the ribosome). During initiation, the nucleotides in the Shine-Dalgarno sequence pair with their complementary nucleotides in the 16S rRNA, allowing the small subunit of the ribosome to attach to the mRNA and positioning the ribosome directly over the initiation codon.

The Initiation of Translation Eukaryotic Initiation System The small subunit of the eukaryotic ribosome, with the help of initiation factors, recognizes the cap and binds there; the small subunit then migrates along (scans) the mRNA until it locates the first AUG codon. The identification of the start codon is facilitated by the presence of a consensus sequence (called the Kozak sequence) that surrounds the start codon:

Lehninger (5E)

Elongation Elongation requires the 70S complex just described tRNAs charged with their amino acids; several elongation factors (EF-Ts, EF-Tu, and EF-G); (4) GTP. A ribosome has three sites that can be occupied by tRNAs; the aminoacyl, or A site, the peptidyl, or P site, and the exit, or E site

Regeneration of EF-Tu-GTP complex to carry out the elongation Lehninger (5E)

How peptide chain grows? Lehninger (5E)

Termination

The Overall Process of Protein Synthesis

RNA–RNA Interactions in Translation (1) The process of translation is rich in RNA–RNA interactions. For example, in bacterial translation, the Shine-Dalgarno consensus sequence at the 5’ end of the mRNA pairs with the 3’ end of the 16S, which ensures the binding of the ribosome to mRNA

Polyribosomes In both prokaryotic and eukaryotic cells, mRNA molecules are translated simultaneously by multiple ribosomes. The resulting structure—an mRNA with several ribosomes attached—is called a polyribosome. Each ribosome successively attaches to the ribosome- binding site at the 5’ end of the mRNA and moves toward the 3’ end; the polypeptide associated with each ribosome becomes progressively longer as the ribosome moves along the mRNA.

A Comparison of Bacterial and Eukaryotic Translation Initiator Amino Acid Temporal and Spatial Differences of Gene Expression Longevity of mRNA Sizes and Compositions Ribosomal Subunits Initiation of Translation Elongation and Termination

The Posttranslational Modifications of Proteins

Translation and Antibiotics

Disruption of peptide bond formation by puromycin.

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