Translation initiation | Translation in prokaryotes lecture 3

Summary

This video explains the initiation phase of protein synthesis in prokaryotes, starting with the charging of tRNA. It covers the formation of the 30S initiation complex, the interaction of ribosomal subunits with mRNA, and the roles of initiation factors like IF1, IF2, and IF3. The process details how the Shine-Dalgarno sequence helps align the ribosome with the start codon on mRNA, allowing the formylated methionine tRNA to bind at the P-site. After this, the 50S subunit joins to form the complete 70S initiation complex, setting the stage for elongation in protein synthesis.

Takeaways

• 😀 The initiation phase of protein synthesis in prokaryotes involves several critical components: mRNA, ribosome subunits (30S and 50S), and charged tRNAs.
• 😀 The process begins with the formation of the 30S initiation complex, which includes initiation factors (IF3, IF1, IF2-GTP).
• 😀 The Shine-Dalgarno sequence in mRNA helps the 30S subunit bind, ensuring proper alignment with the start codon (AUG).
• 😀 The first tRNA, carrying formylmethionine (fMet), binds to the P-site of the ribosome at the start codon, initiating translation.
• 😀 IF3 blocks the E-site, IF1 blocks the A-site, ensuring that only the P-site is available for the first tRNA to bind.
• 😀 The GTP-bound IF2 protein helps bring the first tRNA (fMet) to the P-site, initiating the translation process.
• 😀 Once the first tRNA binds, IF3 is released, allowing for the subsequent assembly of the 70S initiation complex.
• 😀 The 50S ribosomal subunit then binds to the 30S subunit, forming the complete 70S initiation complex.
• 😀 The formation of the 70S initiation complex marks the end of the initiation phase and prepares the ribosome for elongation.
• 😀 The translation process moves from the 5' end to the 3' end of mRNA, with the ribosome sliding along the mRNA one codon at a time during elongation.
• 😀 During elongation, the P-site is initially occupied by the first tRNA, and subsequent tRNAs will bind to the A-site.

Q & A

What is the first stage of protein synthesis in prokaryotes?

The first stage of protein synthesis in prokaryotes is the initiation phase, where the 30S ribosomal subunit assembles with mRNA, tRNA, and initiation factors to form the 30S initiation complex, which then leads to the formation of the 70S initiation complex.

What is the function of charged tRNA in the initiation phase?

Charged tRNA is crucial during initiation because it carries the amino acid (in this case, formylmethionine in prokaryotes) and binds to the ribosome's P site, which is essential for the start of protein synthesis at the start codon of the mRNA.

What is the Shine-Dalgarno sequence and how does it help in initiation?

The Shine-Dalgarno sequence is a complementary sequence found in the mRNA that interacts with the 16S rRNA of the 30S ribosomal subunit. This interaction helps align the ribosome with the start codon, ensuring translation begins at the correct location.

What are the roles of the initiation factors IF1, IF2, and IF3 in protein synthesis initiation?

IF1 binds to the A site and prevents premature tRNA binding, IF2 is a GTP-binding protein that helps bring the first tRNA to the P site, and IF3 prevents premature association of the 50S ribosomal subunit with the 30S subunit.

How does the formylmethionine tRNA contribute to protein synthesis in prokaryotes?

The formylmethionine tRNA is the first tRNA to bind to the P site on the 30S ribosomal subunit, carrying the modified methionine (formylmethionine) to initiate translation at the start codon, which is typically AUG.

What happens to the initiation factors after the 50S subunit binds to the 30S subunit?

After the 50S subunit binds to the 30S subunit, GTP bound to IF2 is hydrolyzed into GDP and phosphate, leading to the release of IF1, IF2, and IF3, which results in the formation of the functional 70S initiation complex.

Why is the P site initially occupied during the initiation phase?

The P site is initially occupied by the first tRNA carrying formylmethionine because it is the site where the start codon interacts with the tRNA to initiate translation. The A and E sites are blocked to ensure that only the P site is used during initiation.

How does the ribosome ensure the correct tRNA binds to the start codon?

The ribosome ensures the correct tRNA binds to the start codon by using the Shine-Dalgarno sequence, which aligns the mRNA properly with the ribosome's rRNA, enabling the first tRNA to bind to the correct start codon (AUG).

What is the role of GTP in the initiation phase?

GTP, bound to IF2, helps in the proper binding of the first tRNA to the P site. After the 50S subunit associates with the 30S subunit, GTP is hydrolyzed to GDP, triggering the release of initiation factors and enabling the formation of the 70S initiation complex.

What occurs after the initiation phase of protein synthesis?

After the initiation phase, the 70S initiation complex is formed, and the ribosome moves along the mRNA to the elongation phase, where aminoacyl-tRNAs enter the A site, peptide bonds are formed, and the polypeptide chain grows.