Translation 3

Defining Stages of Gene Expression

• Transcription:

  • The genetic message in DNA is transcribed into mRNA.

  • mRNA retains the nucleotide sequence format from DNA.

• Translation:

  • Genetic message encoded in RNA bases (A, U, C, G) is translated into amino acid sequences using 20 amino acids.

  • The resulting amino acid sequence defines the protein's structure and properties, which depend on the DNA base sequence.

Requirements for Protein Synthesis

• Essential components include:

  • Template with Sequence: Identifies where decoding begins.

  • Supply of Building Blocks: 20 amino acids.

  • Assembly Mechanism: Ribosomal machinery.

  • Guidance Systems: Rules to define decoding procedures (start and stop locations).

  • Energy: Supplied by GTP and ATP, with each amino acid addition costing approximately 4 ATP.

Role of Ribosomes in Protein Synthesis

• Structure: Ribosomes consist of rRNA and protein.

• Mature mRNA: Serves as the translation template, includes a 5’ cap and poly(A) tail in eukaryotes.

• Aminoacyl-tRNAs Activation: All 20 amino acids must be activated.

• Ancillary Factors Needed: A multitude of enzymes and factors help facilitate translation steps.

mRNA Template in Translation

• Structure:

  • Composition: Composed of ribonucleotides (A, C, G, U).

  • Non-coding regions: 5'-UTR and 3'-UTR.

  • Coding Region: Contains the open reading frame (ORF).

  • Utilizes a triplet code, wherein three RNA bases represent a single amino acid.

  • Codons: Non-overlapping sequences that dictate protein synthesis from initiation to termination.

Ribosome Components and Structure

• Ribosome Composition:

  • Bacterial Ribosomes: 70S = 30S + 50S

  • Eukaryotic Ribosomes: 80S = 40S + 60S

  • Total molecular weight is over 2.5 million Da.

  • Assembled in the nucleolus in eukaryotic cells.

• Ribosomal Sites: Contains three sites for tRNA - A (Aminoacyl), P (Peptidyl), and E (Exit).

Detailed Features of Ribosomal Subunits

• Bacterial 30S Subunit:

  • Comprises 16S rRNA and proteins S1-S19.

  • Contains the decoding center (DC) where mRNA and tRNA interaction occurs.

• Bacterial 50S Subunit:

  • Contains 23S and 5S rRNAs along with proteins L1–L31.

  • Houses the peptidyl transferase center (PTC).

Stages of Translation

Stage 1: Initiation

• Process: Requires specific base pairing between mRNA and tRNA (anticodon).

• Start Codon: AUG recognized by the initiator tRNA.

• Initiation Factors: Proteins that aid in assembling the small ribosomal subunit on mRNA and joining the large ribosomal subunit.

• Key Site: The Kozak sequence serves as the initiation site in eukaryotes.

Stage 2: Elongation

• Elongation Factors (EFs/eEFs): Ensure correct amino acid addition during translation.

• Mechanics: 10-40 amino acids added per second with a low error rate (1 in 10,000).

• Peptide Bond Formation: More than one tRNA can create connections between amino acids in a polypeptide chain.

Stage 3: Termination

• Stop Codons: Triggers the recognition by release factors (RFs/eRFs), leading to the hydrolysis of the polypeptide from the tRNA, completing protein synthesis.

Genetic Code Details

• The genetic code is a triplet code, continuous, and non-overlapping, allowing for encoding of 64 combinations for 20 amino acids, where:

  • One base: Only 4 amino acids possible.

  • Two bases: A maximum of 16 amino acids.

  • Three bases provide sufficient triplets for all 20 amino acids while maintaining the integrity of the coding frame.

The Importance of tRNA

• Structure: tRNA molecules have a cloverleaf configuration with conserved tertiary structure (L-shape).

• Function: Carry specific amino acids to ribosomes, utilizing their anticodon to base pair with mRNA.

• Amino Acid Activation: Enzymatic connection through high-fidelity aminoacyl-tRNA synthetases ensures accurate pairing.

Peptide Bond Formation and Catalysis

• Ribozyme Functionality: The ribosome facilitates peptide bond formation through its rRNA, confirming that rRNA possesses catalytic capabilities.

Wobble Pairing in Translation

• Description: Allows flexibility in base pairing, enabling a single tRNA to recognize multiple codons (i.e., first base of the anticodon may pair with multiple codon options).

• Example: If the first base of the anticodon is inosine (I), it can pair with A, U, or C.

Antibiotics Targeting Translation

• Classification: Antibacterial drugs typically target the bacterial ribosome.

• Examples of Antibiotics:

  • Chloramphenicol: Blocks peptidyl transferase.

  • Erythromycin: Impedes elongation.

  • Tetracycline: Inhibits tRNA binding.

• Effects of Aminoglycosides: Such as streptomycin, can induce errors by binding to the decoding center.

Summary of Protein Synthesis

• Translation Explanation: Synthesizes amino acid chains from the genetic code, transitioning from DNA to protein language.

• Eukaryotic Mechanism Complexity: Guided by multiple factors, including cap-binding proteins and initiation factors; ensures tightly controlled and accurate protein synthesis, emphasizing translational control mechanisms.

• The Central Dogma: This module focuses on the specific molecular processes directing gene expression, encompassing the synthesis, maintenance, and repair of genetic information.

• The Flow of Information:

  • DNA (→ Transcription): The synthesis of RNA under the direction of DNA.

  • RNA (→ Translation): The synthesis of a polypeptide under the direction of mRNA.

• Key Stages: Both transcription and translation follow three primary phases: Initiation, Elongation, and Termination.

Defining Stages of Gene Expression

• Transcription:

  • The genetic message in DNA is transcribed into mRNA (messenger RNA).

  • mRNA serves as the bridge between genes and the proteins they encode. It retains the nucleotide sequence format, with Thymine ($T$) replaced by Uracil ($U$).

• Translation:

  • The process where genetic messages encoded in RNA bases ($A, U, C, G$) are translated into linear sequences of amino acids.

  • There are $20$ standard amino acids. The specific sequence of these amino acids, determined by the mRNA template, dictates the final protein's three-dimensional structure and functional properties.

Requirements for Protein Synthesis

• Essential Components:

  • Template with Sequence: mRNA containing the specific instructions and signals for start and stop.

  • Building Blocks: A supply of all $20$ proteinogenic amino acids.

  • Assembly Machinery: The ribosome, which acts as the site of synthesis.

  • Translational Adaptors: tRNA (transfer RNA) molecules that bridge the gap between codons and amino acids.

  • Guidance Systems: Recognition of specific sequences (Start and Stop codons).

  • Energy Consumption: Translation is energetically expensive. Each amino acid addition requires approximately $4$ high-energy phosphate bonds (from $GTP$ and $ATP$):

  • $2$ ATP equivalents for aminoacyl-tRNA charging.

  • $1$ GTP for codon recognition/tRNA binding at the A-site.

  • $1$ GTP for translocation of the ribosome.

Ribosome Structure and Function

• Composition: Ribosomes are complex ribonuclear machines consisting of ribosomal RNA (rRNA) and proteins.

• Bacterial Ribosomes ($70S$):

  • Small Subunit ($30S$): Comprises $16S$ rRNA and $19$ proteins ($S1-S19$). It contains the Decoding Center (DC) where tRNA anticodons must match mRNA codons.

  • Large Subunit ($50S$): Contains $23S$ and $5S$ rRNAs and approx. $31$ proteins ($L1–L31$). It houses the Peptidyl Transferase Center (PTC) which catalyzes peptide bond formation.

• Eukaryotic Ribosomes ($80S$): Consist of a $40S$ (small) and $60S$ (large) subunit. Assembled in the nucleolus.

• Main Ribosomal Sites:

  1. A Site (Aminoacyl): Holds the incoming tRNA carrying the next amino acid.

  2. P Site (Peptidyl): Holds the tRNA carrying the growing polypeptide chain.

  3. E Site (Exit): The site where discharged tRNAs leave the ribosome.

The mRNA Template and Genetic Code

• Structure of Mature mRNA:

  • $5'$ Cap and $3'$ Poly(A) Tail: Essential for stability and initiation in eukaryotes.

  • $5' ext{-UTR}$ and $3' ext{-UTR}$: Untranslated regions that regulate stability and translation efficiency.

  • Coding Region (ORF): The Open Reading Frame starts at the initiation codon and ends at the stop codon.

• The Genetic Code:

  • Triplet Nature: Codons consist of three nucleotides. This allows for $4^3 = 64$ possible combinations, enough to cover $20$ amino acids plus stop signals.

  • Degeneracy/Redundancy: Multiple codons can code for the same amino acid (e.g., Leucine is coded by $6$ different codons), but the code is unambiguous (each codon specifies only one amino acid).

  • Non-overlapping and Continuous: The code is read sequentially without skipping bases.

The Three Stages of Translation in Detail

1. Initiation

• Pre-initiation: Small ribosomal subunit binds to mRNA.

  • In Prokaryotes, the subunit binds to the Shine-Dalgarno sequence upstream of the AUG.

  • In Eukaryotes, the subunit binds to the $5' ext{-cap}$ and scans for the Kozak sequence ($ACCAUGG$).

• Initiatory tRNA: Always carries methionine (formyl-methionine in bacteria) and binds directly to the P-site.

• Assembly: Initiation Factors ($IFs$ or $eIFs$) facilitate the docking of the large ribosomal subunit.

2. Elongation

• Codon Recognition: Aminoacyl-tRNA binds to the A-site (assisted by elongation factors like $EF ext{-Tu}$).

• Peptide Bond Formation: The PTC in the large subunit catalyzes the transfer of the polypeptide chain from the P-site tRNA to the amino acid on the A-site tRNA.

• Translocation: The ribosome moves one codon forward along the mRNA ($5' → 3'$), moving the A-site tRNA to the P-site and the P-site tRNA to the E-site. This requires $EF ext{-G}$ (prokaryotes) or $eEF2$ (eukaryotes) and $GTP$ hydrolysis.

3. Termination

• Stop Codons: Recognition of $UAA, UAG, ext{or } UGA$ in the A-site.

• Release Factors ($RFs$): Protein factors that mimic tRNA structure. They enter the A-site and trigger the hydrolysis of the bond between the polypeptide and the tRNA in the P-site.

• Dissociation: The ribosome complex dissociates into its subunits.

Transfer RNA (tRNA) and Wobble Pairing

• tRNA Structure: Tertiary L-shaped structure. The Anticodon loop pairs with mRNA, and the $3'$ end (CCA tail) carries the amino acid.

• Aminoacyl-tRNA Synthetases: Enzymes that "charge" tRNAs. There is typically one enzyme for each amino acid. They ensure high fidelity (accuracy) in protein synthesis.

• Wobble Hypothesis: The first base of the anticodon (pairing with the third base of the codon) has flexible pairing rules. This explains why cells do not need $61$ different tRNAs for the $61$ sense codons.

  • Inosine ($I$): A modified base in tRNA that can pair with $U, C, ext{or } A$.

Antibiotic Inhibition of Translation

• Antibiotics: Many target differences between bacterial ($70S$) and eukaryotic ($80S$) ribosomes to provide selective toxicity.

  • Chloramphenicol: Inhibits the Peptidyl Transferase Center of the $50S$ subunit.

  • Erythromycin: Binds to the $50S$ subunit and prevents translocation (exit tunnel blockage).

  • Tetracyclines: Prevent the binding of aminoacyl-tRNA to the A-site of the $30S$ subunit.

  • Aminoglycosides (e.g., Streptomycin): Bind to the $30S$ subunit decoding center, causing mRNA misreading and truncated proteins.