BIOCHEM FINAL - Initiation

Ribosomes are the sites of protein synthesis. They are made of two subunits (large and small), each containing rRNA and proteins

What are ribosomes and their main components?

rRNA performs the chemical reactions of protein synthesis, not the proteins

Which component of the ribosome performs the catalytic activity in protein synthesis?

mRNA passes through the cleft between the large and small subunit during translation

How does mRNA interact with the ribosome?

rRNA structure is highly conserved across all organisms and is thought to be one of the earliest evolved molecules

Why is rRNA considered evolutionary significant?

70S ribosome (50S large + 30S small)

What are the sizes of prokaryotic ribosomes?

80S ribosome (60S large and 40S small)

what are the sizes of eukaryotic ribosomes?

A ribosomal binding site that base pairs with the 3’ end of 16S rRNA, aligning the AUG start codon with the P-site

What is the Shine-Dalgarno sequence in bacterial mRNA?

It is only used for starting translation, not for elongation

What is unique about the bacterial initiator tRNA?

IF1 blocks the A-site to prevent incorrect tRNA binding

What is the role of IF1 in bacterial initiation?

IF3 binds the E-site and prevents and prevents the 50S subunit from joining prematurely

what is the role of IF3 in bacterial initiation?

IF2-GTP binds the initiator tRNA and brings it to the P-site of the 30S subunit

what is the role of IF2-GTP in bacterial initiation?

Hydrolysis of GTP on IF2 trigger:

-release of IF1, IF2, IF3

-binding of the 50S large subunit

-initiation is then complete

How is translation started in bacteria

It is the small 40S subunit with:

-initiator tRNA in P-site

-elF1 (E-site)

-elF1A (A-site)

-elF3 (stabilization of small subunit)

-elF2-GTP (brings tRNA)

-elF5B-GTP (stabilization)

What is the 43S pre-initiation complex in eukaryotes?

elF4F binds the 5’ cap and poly-A tail of mRNA and brings it to the 43S complex, forming the 48S complex

How is mRNA recruited in eukaryotic initiation?

The 48S complex scans for the AUG start codon using ATP, because elF4F has helicase activity to unwind RNA structures

How does scanning of mRNA occur in eukaryotes?

Once AUG is found, elF2 and elF5B hydrolyze GTP. all elFs are released, the 60S large subunit binds. Initiation is complete

What triggers commitment to start translation in eukaryotes?

In the P-site of the small ribosomal subunit

Where does the initiator tRNA sit in both bacteria and eukaryotes?

Only after the start codon (AUG) is located and initiation factors have been released

When does the large ribosomal subunit join in translation?