Translation Process in Prokaryotes and Eukaryotes

Translation Process in Prokaryotes and Eukaryotes

• The translation process occurs in both prokaryotes and eukaryotes, with distinct mechanisms.
• In prokaryotes, the initiator tRNA initiates the polypeptide chain.
• Eukaryotic initiation involves scanning the small ribosomal subunit for initiation sites.
• Regulation mechanisms control the accuracy of translation.
• The ribosome has three tRNA-binding sites (A, P, and E).
• tRNA binding and translocation occur between A, P, and E sites to continue polypeptide synthesis.

mRNA Translation Circuit

• Cytoplasmic mRNA has three locations: polyribosomes, stress granules, and P bodies.
• The mRNA circuit demonstrates the regulation of mRNA translation.
• m7Gppp, 4E, 4G, initiation complex, rounds of translation, PAB, and other factors impact translation initiation.
• P-bodies, recruit Dcp1/Dcp2, sm1-7, promote mRNA decay, and stress granules.

mRNA Translation Process

• The ribosome has large and small subunits.
• Amino acids are brought to the ribosome by tRNA molecules.
• The anticodon of tRNA pairs with the codon of mRNA.
• Peptide bonds form between amino acids.

Dimensions of Translation Apparatus

• Ribosomes are composed of rRNA and proteins.
• Bacterial ribosomes (70S) have 50S and 30S subunits.
• Mammalian ribosomes (80S) have 60S and 40S subunits.
• The different subunits are made up of various rRNAs and proteins.

Three Stages of Translation

• Prokaryotic initiation, elongation, and termination require distinct factors (IFs, EFs, and RFs).
• Eukaryotic initiation, elongation, and termination employ different factors (elFs, eEFs, and eRFs).

Controlling Translation Accuracy

• Errors in translation occur at a rate of 10^-4 to 10^-5 per codon.
• Mechanisms such as proofreading by aminoacyl-tRNA synthetases reduce errors.
• Ribosomal errors can arise from incorrect tRNA binding during polypeptide chain elongation.

Prokaryotic Translation Initiation

• Prokaryotic translation begins at the RBS (ribosome binding site).
• The specific region on mRNA contains the Shine-Dalgarno sequence.
• The AUG start codon is preceded by the Shine-Dalgarno sequence upstream.
• N-formylmethionyl-tRNA (tRNAfMet) initiates bacterial polypeptide synthesis.

Eukaryotic Translation Initiation

• Eukaryotic mRNAs have a methylated 5' cap and a poly(A) tail at the 3' end.
• The small subunit scans the 5'UTR to locate the Kozak consensus sequence upstream of the AUG start codon.
• The initiator tRNA (met-tRNAi) forms a complex with initiation factors.

Eukaryotic Translation Initiation: Formation of the 43S Pre-initiation Complex

• Ribosomal subunits participate in a cyclical translation initiation process.
• Initiation factors assist in subunit joining and recycling.
• The 43S pre-initiation complex involves various factors and the tRNA (Met-tRNAi).

Eukaryotic Translation Initiation: Association with Cap

• The small subunit binds to the 5' cap of mRNA.
• Factors and proteins interact with the mRNA to position the ribosome.
• RNA helicase facilitates interaction with the mRNA 5´cap and poly(A) tail by interacting with the mRNA.

Eukaryotic Translation Initiation: Formation of the 48S Initiation Complex

• Codon recognition in eukaryotes involves mRNA scanning.
• Initiation factors assist in positioning the ribosome over the start codon,
• Initiator tRNA is positioned in P-site.

Eukaryotic Translation Initiation is Complete

• Initiation factors facilitate the large subunit joining.
• Hydrolysis of GTP releases factors, and the 80S ribosome forms.

Overview of Eukaryotic Translation

• Eukaryotic translation initiation can occur from non-AUG codons.
• Both cap-dependent and –independent methods are present.
• CUG, UUG are considered less frequent start codons.

Internal Ribosomal Entry Sites (IRES)

• IRES are cis-acting sequences that allow cap-independent translation initiation.
• IRES are present in the 5′ UTR of some viral and cellular mRNAs.
• IRES elements can promote internal initiation of translation initiation through RNA-RNA and/or RNA-protein interactions.

Leaky Scanning

• Translation can be initiated from multiple start codons, even if they are not ideal based on Kozak sequence in mRNA.
• Initiation can skip the first AUG, going to the following one if it is present in the proximity.

EF-Tu(Prok)/eEF1(Alpha)

• EF-Tu(Prok)/eEF1(Alpha) is important for tRNA binding and translocation, assisting in peptide bond formation.
• GTP hydrolysis is essential for this process.
• Initiation tRNA occupies P-site, which allows second amino acid to enter A-site.

Prokaryotic vs Eukaryotic Translation Factors

• Factors required for prokaryotic and eukaryotic processes play essential roles.
• The comparison table summarizes the corresponding factors in each process.

Translation Termination

• Release factors (RFs) recognize stop codons.
• The polypeptide chain detaches, and ribosome subunits disassociate.
• Molecular mimicry is observed as stop codons cause release factors, resembling tRNA in their structure.

Signal Recognition Particle (SRP)

• SRP directs proteins to the ER during synthesis.
• It recognizes signal peptides during polypeptide translation initiation.
• SRP facilitates the translocation of proteins across the endoplasmic reticulum membrane.

Regulation of mRNA Translation

• Regulation elements in mRNA can affect translation.
• Secondary and tertiary structures, such as stem and loops, can regulate translation
• Regulatory factors (like initiation and elongation factors) play a significant role in the regulation of mRNA translation.

Regulation of mRNA Translation: Prokaryotes

• Operons, structure and stem-loops influence translation initiation.
• Temperature changes can trigger expression of cold shock proteins (CSPs).
• CSPs maintain mRNA conformation for stable translation
• RNase R degrades mRNA during recovery.

Regulation of mRNA Translation: Eukaryotes

• Inhibition factors interfere with elF4F assembly reducing translation.
• 4E-BPs, interacting with elF4E, modulate translation initiation.
• mTOR, a kinase, regulates translation by phosphorylation of 4E-BPs

mRNA Cycle: Cycling of Eukaryotic mRNAs

• mRNA can transition between transcription, translation, and degradation subcellular compartments.
• P-bodies and stress granules are important structures for mRNA processing and translation regulation.

Stress Granules

• Stress granules are aggregates of proteins, mRNAs, and other components.
• They represent a cellular response to stress, preserving mRNA conformation and translational inhibition.
• They are a type of quality control mechanism

Antisense Oligonucleotides (ASOs)

• ASOs can be used as therapeutic tools for gene regulation.
• They can bind mRNA, leading to degradation, splicing changes, or sequestration of mRNA by interfering directly with the process.

Explore the fundamental mechanisms of translation in both prokaryotic and eukaryotic systems. Understand the roles of tRNA, ribosomal sites, and the regulatory factors involved in mRNA translation. This quiz covers essential concepts critical for molecular biology.