Eukaryotic Translation

Please feel free to add if I may have missed anything or correct if I’m even partially wrong.

tRNA molecule
• Single chain, contains 73-93 ribonucleotides
• Contains many unusual bases such as inosine, pseudouridine
• tRNA is L shaped
• 5’ end is phosphorylated
• 3’ end ends in CCA and contains the amino acid attachment, it is at one end of the L
• The other end of the L, far from the amino acid end, is the anticodon loop

The process of translation, like transcription, is also divided into three phases:
initiation, elongation and termination. These three phases are regulated by initiation, elongation and termination factors respectively.

Initiation

Initiator tRNA (tRNAi) that carries methionine is the only tRNA capable of initiating translation. An initiation complex called 43S, comprising methionine tRNAi, the small 40S ribosomal subunit, and initiation factors such as eIF2. The 43S complex is recruited to the 5’ end of the mRNA by eIF4E. This complex now scans the mRNA in the 5’ to 3’ direction to find the first 5’-AUG-3’. Scanning is an ATP dependent process. As soon as the met-tRNAi finds the first AUG, the larger ribosomal subunit is recruited and this recruitment is mediated by eIF5. Assembly of the large ribosomal subunit completes the initiation step. The large subunit has 3 binding sites, E, P and A and the first codon (AUG) is aligned at the P site.

Elongation

Elongation begins with the delivery of an amino-acyl tRNA (corresponding to the appropriate codon on the mRNA) to the A site on the ribosome by EF-Tu and this is followed by GTP hydrolysis. A peptide bond, catalyzed by peptidyl transferase, is formed between methionine and the aminoacyl tRNA by the transfer of methionine to the A site, leaving the deacylated tRNA at the P site. The next step of elongation is translocation, where, the deacylated tRNA moves to the E site, the dipeptidyl-tRNA (met + aminoacyl tRNA) moves to the P site and the mRNA moves forward by 3 bases, thereby aligning the next codon for the appropriate aminoacyl tRNA. Translocation is mediated by elongation factor G. A and E sites cannot be occupied at the same time, therefore, as soon as the A site is occupied, the E site containing the deacylated tRNA is emptied. Elongation proceeds in this fashion until a stop codon is encountered.

Termination

Normally, tRNAs do not have anticodons corresponding to the stop codons (UAA, UAG or UGA. At termination the polypeptide chain is at the P site and the stop codon is at the A site. Stop codons are recognized by proteins called release factors (RFs) or termination factors. Peptidyl transferase is activated when an RF binds to a termination codon at the A site. The activated peptidyl transferase hydrolyzes the bonds between the polypeptide and the tRNA at the P site. The released polypeptide chain, tRNA and mRNA leave the ribosome in that order. The ribosome dissociates into its subunits ready for another round of protein synthesis.

Summarizing eukaryotic protein translation

• mRNA is always translated in the 5’ to 3’ direction
• proteins are synthesized in the amino to carboxyl direction
• several ribosomes can simultaneously translate an mRNA molecule and such an mRNA molecule (with many ribosomes attached) is called a polysome or a polyribosome
• amino acids are added sequentially to the carboxyl end of a polypeptide chain
• aminoacyl tRNAs are the activated precursors in which the carboxyl group of an amino acid is attached to the 3’ hydroxyl group of a tRNA
• the above step is catalyzed by an aminoacyl tRNA synthetase and is driven by ATP
• initiator tRNA, met-tRNAi, occupies peptidyl (P) site, the next aminoacyl tRNA, added during elongation, occupies the aminoacyl (A) site
• peptide bond is formed between carboxyl group of met and aminoacyl tRNA
• dipeptidyl tRNA moves from A to P site
• deacylated tRNAi moves to E (exit) site and leaves ribosome
• a new aminoacyl tRNA occupies A site
• elongation proceeds until stop codon is encountered
• stop codon (UGA, UAA, or UAG) recognized by release factors that facilitate release of the completed polypeptide from the ribosome