Translation In Prokaryotes II Protein Synthesis In Prokaryotes

 Translation in prokaryotes is the process by which the genetic information encoded in the messenger RNA (mRNA) is translated into a protein sequence by the ribosome. Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles, so translation occurs in the cytoplasm.

The process of translation in prokaryotes involves three main stages:

·         Initiation,

·         Elongation,

·         Termination.

 

Initiation:

The initiation process of translation in prokaryotes involves several steps that prepare the ribosome to begin translating the mRNA sequence. Here is a detailed overview of the initiation process:

1. Binding of the small ribosomal subunit to the mRNA: The small ribosomal subunit binds to the mRNA at a specific site called the Shine-Dalgarno (SD) sequence, which is a short sequence of nucleotides located upstream of the start codon (AUG). The SD sequence base-pairs with the anti-SD sequence in the 16S rRNA of the small ribosomal subunit, ensuring proper alignment of the mRNA with the ribosome.
2. Recruitment of the initiation factors: The initiation factors IF1, IF2, and IF3 bind to the small ribosomal subunit, helping to stabilize the complex and prepare it for initiation. IF1 and IF3 prevent premature binding of the large ribosomal subunit, while IF2 recruits the initiator tRNA and helps it bind to the start codon.
3. Selection of the initiator tRNA: The initiator tRNA, which carries the amino acid methionine (Met), is recognized by the initiation factor IF2 and loaded onto the P site of the ribosome. In prokaryotes, the initiator tRNA is fMet-tRNAfMet, which is a modified form of tRNA that specifically recognizes the AUG start codon.
4. Binding of the large ribosomal subunit: Once the initiation complex is formed, the large ribosomal subunit binds to the small subunit, forming the complete ribosome. The initiator tRNA is positioned at the P site of the ribosome, while the A site is vacant and ready to receive the next aminoacyl-tRNA.
5. GTP hydrolysis and release of initiation factors: The initiation factors IF1, IF2, and IF3 are no longer needed and are released from the ribosome. The hydrolysis of GTP to GDP by IF2 provides the energy needed for the initiation factors to be released from the ribosome.

Overall, the initiation process of translation in prokaryotes is a complex and highly regulated process that requires the participation of several protein factors and RNAs to ensure proper alignment of the mRNA with the ribosome, selection of the correct initiator tRNA, and formation of the complete ribosome.

Elongation:

The elongation process of translation in prokaryotes involves the step-by-step addition of amino acids to the growing polypeptide chain. Here is a detailed overview of the elongation process:

1. Recognition of the codon: The ribosome moves along the mRNA in the 5' to 3' direction, reading each codon in the A site of the ribosome. The anticodon of the incoming aminoacyl-tRNA base pairs with the codon in the mRNA, ensuring that the correct amino acid is added to the growing polypeptide chain.
2. Peptide bond formation: Once the correct aminoacyl-tRNA is bound to the A site, the ribosome catalyzes the formation of a peptide bond between the carboxyl group of the amino acid in the P site and the amino group of the amino acid in the A site. This reaction is catalyzed by the peptidyl transferase activity of the ribosome, which is carried out by the 23S rRNA.
3. Translocation: After peptide bond formation, the ribosome moves one codon down the mRNA, shifting the tRNAs to the P and E sites of the ribosome. The tRNA in the P site now carries the nascent polypeptide chain, while the tRNA in the A site is now empty and ready to receive the next aminoacyl-tRNA.
4. Release of the uncharged tRNA: The uncharged tRNA in the E site is released from the ribosome, and it can now be reloaded with another amino acid by aminoacyl-tRNA synthetase.
5. Repeated elongation cycles: The process of codon recognition, peptide bond formation, translocation, and release of the uncharged tRNA is repeated, adding one amino acid at a time to the growing polypeptide chain. This process continues until a stop codon is encountered in the A site.

Overall, the elongation process of translation in prokaryotes is a highly coordinated and efficient process that involves the participation of several protein factors and RNAs to ensure the accurate and efficient addition of amino acids to the growing polypeptide chain.

Termination:

The termination process of translation in prokaryotes involves the recognition of a stop codon in the mRNA and the release of the completed polypeptide chain from the ribosome. Here is a detailed overview of the termination process:

1. Recognition of the stop codon: When a stop codon (UAA, UAG, or UGA) is encountered in the A site of the ribosome, it does not code for any amino acid and instead signals the end of the polypeptide chain. The stop codon is recognized by a release factor protein, which mimics the shape of a tRNA molecule.
2. Peptide bond hydrolysis: The release factor protein binds to the A site of the ribosome, causing the peptidyl transferase activity of the ribosome to hydrolyze the peptide bond between the completed polypeptide chain and the tRNA in the P site. This releases the completed polypeptide chain from the ribosome.
3. Disassembly of the ribosome: The ribosome dissociates into its individual subunits, ready to begin another round of translation. The released mRNA can be degraded or recycled for another round of translation.

Overall, the termination process of translation in prokaryotes is a highly coordinated and efficient process that involves the recognition of a stop codon, the binding of a release factor protein, and the hydrolysis of the peptide bond between the completed polypeptide chain and the tRNA in the P site. The release of the completed polypeptide chain from the ribosome and the disassembly of the ribosome allow for the efficient production of multiple copies of a given protein.