Chapter 15: The Genetic Code and Translation

The Relationship Between Genes, Enzymes, and Proteins

• Foundational Discoveries:

  • In 1941, George Beadle and Edward Tatum proposed the one gene, one enzyme hypothesis, which postulated that genes function by directly encoding enzymes, with each distinct gene encoding a specific single enzyme.

  • Subsequent molecular research refined this concept into the one gene, one polypeptide hypothesis, acknowledging that many functional proteins and enzymes consist of multiple distinct polypeptide subunits encoded by separate genes.

• Neurospora crassa as a Model Organism:

  • Neurospora crassa (orange bread mold) was utilized to dissect the biochemical relationships between genes and proteins.

  • Wild-type Neurospora is a prototroph capable of growing on minimal medium—a simple mixture containing inorganic salts, a carbon source (such as sucrose or glucose), and the vitamin biotin. The fungus utilizes these minimal ingredients to synthesize all necessary organic compounds, including all 20 standard amino acids, purines, pyrimidines, and complex vitamins.

  • Auxotrophs are nutritional mutant strains that possess metabolic defects and cannot survive on minimal medium unless supplemented with specific organic nutrients that they can no longer synthesize autonomously.

• Isolation and Identification of Auxotrophic Mutants:

  • Step 1: Mutagenesis: A culture of Neurospora asexual spores was irradiated with X-rays to induce genetic mutations.

  • Step 2: Culturing on Complete Medium: Individual surviving spores were transferred to test tubes containing complete medium (minimal medium supplemented with all 20 amino acids, vitamins, and purines/pyrimidines). This ensured that both wild-type and mutant strains grew and reproduced.

  • Step 3: Screening on Minimal Medium: Spores from each individual culture were subsequently transferred to tubes containing solely minimal medium.

  • Step 4 & 5: Phenotypic Selection:

  • Strains carrying auxotrophic mutations failed to grow on minimal medium.

  • Strains that successfully grew on minimal medium were prototrophic (lacked auxotrophic mutations).

  • Step 6: Diagnostic Supplementation: Spores from each candidate mutant strain were transferred into a battery of separate tubes containing minimal medium supplemented with only one specific amino acid out of the 20 biological amino acids.

  • Step 7: Identifying Defective Pathways: If growth occurred exclusively in the tube containing a specific nutrient (for example, arginine), the mutant was confirmed to have a lesion specifically disrupting the biosynthetic pathway of that nutrient.

• Dissecting Biochemical Pathways of Amino Acid Synthesis:

  • Adrian Srb and Norman Horowitz expanded Beadle and Tatum's work by analyzing a collection of arginine auxotrophic mutants to order the enzymes and intermediates in the arginine biosynthetic pathway.

  • The known biochemical pathway for arginine synthesis proceeds through distinct intermediates:     $\text{Precursor compound} \xrightarrow{\text{Enzyme A (Gene A)}} \text{Ornithine} \xrightarrow{\text{Enzyme B (Gene B)}} \text{Citrulline} \xrightarrow{\text{Enzyme C (Gene C)}} \text{Arginine}$

  • Mutants were categorized into three distinct genetic complementation groups based on their growth response to minimal medium supplemented with intermediates:

  • Group I Mutants (Defective in Gene A / Enzyme A): Grew on minimal medium supplemented with ornithine, citrulline, or arginine. Because the metabolic block occurs prior to ornithine synthesis, supplying any downstream intermediate bypasses the genetic block.

  • Group II Mutants (Defective in Gene B / Enzyme B): Grew on minimal medium supplemented with either citrulline or arginine, but failed to grow when supplemented with ornithine. The block occurs between ornithine and citrulline; hence, ornithine cannot be converted to downstream products.

  • Group III Mutants (Defective in Gene C / Enzyme C): Grew exclusively on minimal medium supplemented with arginine. The block occurs between citrulline and arginine, meaning neither upstream precursor can rescue growth.

• Summary of Growth Patterns for Arginine Auxotrophs (Table 15.1):

  • Wild type: Grows on minimal medium alone ($+$), and grows when supplemented with ornithine ($+$), citrulline ($+$), or arginine ($+$).

  • Group I: Minimal medium alone ($-$); supplemented with ornithine ($+$); supplemented with citrulline ($+$); supplemented with arginine ($+$).

  • Group II: Minimal medium alone ($-$); supplemented with ornithine ($-$); supplemented with citrulline ($+$); supplemented with arginine ($+$).

  • Group III: Minimal medium alone ($-$); supplemented with ornithine ($-$); supplemented with citrulline ($-$); supplemented with arginine ($+$).

Protein Structure and Amino Acid Chemistry

• Basic Molecular Architecture of Amino Acids:

  • Proteins are linear polymers assembled from 20 standard amino acids joined together through covalent peptide bonds.

  • Every standard amino acid possesses a central tetrahedral alpha-carbon ($\text{C}_\alpha$) bonded to four distinct functional groups:

  1. An amino group ($-\text{NH}_3^+$ or $-\text{NH}_2$).

  2. A carboxyl group ($-\text{COO}^-$ or $-\text{COOH}$).

  3. A hydrogen atom ($-\text{H}$).

  4. A variable side chain designated as the radical group ($-\text{R}$), which confers unique chemical properties and determines the identity of each amino acid.

• Classification of the 20 Amino Acids by R-Group Properties:

  • Nonpolar, Aliphatic R Groups (Hydrophobic):

  • Glycine ($\text{Gly, G}$): Side chain is a single hydrogen atom ($-\text{H}$).

  • Alanine ($\text{Ala, A}$): Side chain is a methyl group ($-\text{CH}_3$).

  • Valine ($\text{Val, V}$): Branched side chain ($-\text{CH(CH}_3)_2$).

  • Leucine ($\text{Leu, L}$): Branched side chain ($-\text{CH}_2\text{-CH(CH}_3)_2$).

  • Isoleucine ($\text{Ile, I}$): Hydrocarbon isomer of leucine ($-\text{CH(CH}_3)\text{-CH}_2\text{-CH}_3$).

  • Methionine ($\text{Met, M}$): Thioether side chain ($-\text{CH}_2\text{-CH}_2\text{-S-CH}_3$).

  • Polar, Uncharged R Groups (Hydrophilic):

  • Serine ($\text{Ser, S}$): Hydroxyl group ($-\text{CH}_2\text{OH}$).

  • Threonine ($\text{Thr, T}$): Hydroxyl group ($-\text{CH(OH)-CH}_3$).

  • Cysteine ($\text{Cys, C}$): Sulfhydryl/thiol group ($-\text{CH}_2\text{-SH}$), capable of forming covalent disulfide bonds.

  • Proline ($\text{Pro, P}$): Cyclic imino acid whose aliphatic side chain bonds back to the amino group, forming a rigid ring structure ($-\text{CH}_2\text{-CH}_2\text{-CH}_2-$).

  • Asparagine ($\text{Asn, N}$): Amide side chain ($-\text{CH}_2\text{-CONH}_2$).

  • Glutamine ($\text{Gln, Q}$): Amide side chain ($-\text{CH}_2\text{-CH}_2\text{-CONH}_2$).

  • Aromatic R Groups:

  • Phenylalanine ($\text{Phe, F}$): Nonpolar benzene ring ($-\text{CH}_2\text{-C}_6\text{H}_5$).

  • Tyrosine ($\text{Tyr, Y}$): Phenolic hydroxyl ring ($-\text{CH}_2\text{-C}_6\text{H}_4\text{-OH}$).

  • Tryptophan ($\text{Trp, W}$): Bicyclic indole ring ($-\text{CH}_2\text{-indole}$).

  • Positively Charged (Basic) R Groups:

  • Lysine ($\text{Lys, K}$): Primary aliphatic amine tail ($-(\text{CH}_2)_4\text{-NH}_3^+$).

  • Arginine ($\text{Arg, R}$): Positively charged guanidinium group ($-(\text{CH}_2)_3\text{-NH-C(NH}_2)^+=\text{NH}_2$).

  • Histidine ($\text{His, H}$): Ionizable imidazole ring ($-\text{CH}_2\text{-imidazole}$).

  • Negatively Charged (Acidic) R Groups:

  • Aspartate ($\text{Asp, D}$): Carboxylate group ($-\text{CH}_2\text{-COO}^-$).

  • Glutamate ($\text{Glu, E}$): Carboxylate group ($-\text{CH}_2\text{-CH}_2\text{-COO}^-$).

• Peptide Bond Formation and Polypeptide Polarity:

  • Amino acids polymerize via a condensation (dehydration) reaction between the carboxyl carbon of one amino acid and the alpha-amino nitrogen of the adjacent amino acid.

  • The resulting covalent linkage ($-\text{CO-NH}-$) is termed a peptide bond, releasing a molecule of water ($\text{H}_2\text{O}$).

  • A polypeptide chain possesses intrinsic structural directionality (polarity):

  • Amino Terminus (N-terminus): Retains a free, unbonded protonated amino group ($-\text{NH}_3^+$).

  • Carboxyl Terminus (C-terminus): Retains a free, unbonded ionized carboxyl group ($-\text{COO}^-$).

  • Synthesis always proceeds sequentially from the N-terminus toward the C-terminus.

• Hierarchy of Protein Structural Organization:

  • Primary Structure: The linear sequence of amino acids covalently linked by peptide bonds, specified directly by the nucleotide sequence of the mRNA.

  • Secondary Structure: Local spatial folding of the polypeptide backbone stabilized primarily by hydrogen bonds between the carbonyl oxygen and amide hydrogen atoms of the peptide backbone. Common configurations include the $\alpha$-helix and the $\beta$-pleated sheet.

  • Tertiary Structure: The complete three-dimensional spatial conformation of a single polypeptide chain, driven and stabilized by interactions among variable R groups (hydrophobic interactions, hydrogen bonds, ionic electrostatic bonds, van der Waals forces, and covalent disulfide bridges).

  • Quaternary Structure: The stable multi-subunit complex formed by the structural association of two or more independent polypeptide chains (e.g., hemoglobin, multi-subunit enzymes).

  • Determinant Principle: The higher-order secondary and tertiary structures of a native protein are fundamentally determined by its primary amino acid sequence.

Characteristics and Deciphering of the Genetic Code

• Fundamental Features of the Genetic Code:

  • Codon Definition: A sequence of three consecutive nucleotides in mRNA that specifies a single amino acid or signals the termination of translation.

  • Triplet Coding Capacity: Because RNA contains 4 distinct ribonucleotide bases (A, U, G, C), a triplet code generates $4^3 = 64$ unique codons. This is more than sufficient to specify the 20 biological amino acids.

  • Sense Codons: 61 of the 64 codons encode specific amino acids.

  • Termination (Stop or Nonsense) Codons: 3 codons ($\text{UAA}$, $\text{UAG}$, and $\text{UGA}$) do not encode amino acids; they signal the termination of translation.

  • Initiation (Start) Codon: The triplet $\text{AUG}$ serves as the principal initiation signal, encoding Methionine ($\text{Met}$).

  • Nonoverlapping Nature: The genetic code is read sequentially in discrete triplets without overlapping nucleotides; a single nucleotide is part of only one codon.

  • Reading Frame: A specific linear sequence of nucleotides can be read in three potential reading frames depending on where decoding begins. The precise placement of the start codon sets the correct reading frame.

• Experimental Deciphering of Codon Identities:

  • Homopolymer Assays (Nirenberg and Matthaei):

  • Marshall Nirenberg and Heinrich Matthaei synthesized synthetic RNA homopolymers using the enzyme polynucleotide phosphorylase, which links ribonucleoside diphosphates randomly without requiring a DNA template.

  • Homopolymers contained only a single ribonucleotide type: $\text{poly(U)}$, $\text{poly(A)}$, or $\text{poly(C)}$.

  • Synthetic RNAs were added to cell-free translation extracts containing bacterial ribosomes, tRNAs, translation factors, energy sources, and 20 tubes each containing 19 unlabeled amino acids plus 1 radioactively labeled amino acid.

  • Reaction products were precipitated with trichloroacetic acid and collected on a filter to detect radioactive peptide synthesis.

  • Results:

    • $\text{Poly(U)}$ mRNA produced polyphenylalanine, demonstrating that $\text{UUU}$ encodes Phenylalanine ($\text{Phe}$).

    • $\text{Poly(A)}$ mRNA produced polylysine, demonstrating that $\text{AAA}$ encodes Lysine ($\text{Lys}$).

    • $\text{Poly(C)}$ mRNA produced polyproline, demonstrating that $\text{CCC}$ encodes Proline ($\text{Pro}$).

    • $\text{Poly(G)}$ formed complex secondary guanine-quartet structures that could not be translated in this system.

• Random Copolymer Assays:

  • Ribonucleotide mixtures in defined proportions (e.g., 5 parts U to 1 part G) were polymerized, generating random copolymers with predictable triplet frequencies. By correlating the statistical frequencies of incorporated amino acids with calculated codon frequencies, additional codon assignments were deduced, although exact nucleotide orders could not be confirmed.

• Ribosome-Bound tRNA Assay (Nirenberg and Leder):

  • Very short synthetic trinucleotide RNAs of known sequence were synthesized and incubated with purified ribosomes and aminoacyl-tRNAs (with one amino acid radioactively labeled per reaction).

  • Ribosomes bound the synthetic trinucleotide and the cognate charged tRNA possessing a complementary anticodon.

  • The mixture was passed through a nitrocellulose filter membrane. Large ribosome complexes (and any bound trinucleotide-tRNA) were retained on the filter, while small unbound tRNAs passed through.

  • Assaying the radioactive amino acid retained on the filter directly matched specific triplet codons to their designated amino acids (e.g., synthetic $\text{GUU}$ retained valine-charged tRNA, confirming $\text{GUU}$ specifies Valine). This technique resolved more than 50 of the 64 codons.

• Complete Genetic Code Dictionary:

• Degeneracy and the Wobble Hypothesis:

  • Degenerate Code: The code contains redundant information; most amino acids are specified by multiple synonymous codons (e.g., Leucine, Serine, and Arginine are each encoded by 6 different codons).

  • Isoaccepting tRNAs: Different tRNA species that attach to the same amino acid but possess distinct anticodon sequences.

  • Crick's Wobble Hypothesis (1966):

  • Organisms generally contain between 30 and 50 distinct tRNA species to decode the 61 sense codons.

  • Pairing between the first two bases of the mRNA codon (read in the $5' \rightarrow 3'$ direction) and the complementary second and third bases of the tRNA anticodon (read in the $3' \rightarrow 5'$ direction) follows strict standard Watson-Crick hydrogen bonding ($\text{A-U}$ and $\text{G-C}$).

  • Hydrogen bonding at the third position of the codon ($3'$ end) and the first position of the anticodon ($5'$ end) exhibits structural flexibility, or "wobble".

• Wobble Base-Pairing Rules (Table 15.2):

  • Anticodon First Base ($5'$ end) $\text{C}$ pairs with Codon Third Base ($3'$ end) $\text{G}$.

  • Anticodon First Base ($5'$ end) $\text{G}$ pairs with Codon Third Base ($3'$ end) $\text{U}$ or $\text{C}$.

  • Anticodon First Base ($5'$ end) $\text{A}$ pairs with Codon Third Base ($3'$ end) $\text{U}$.

  • Anticodon First Base ($5'$ end) $\text{U}$ pairs with Codon Third Base ($3'$ end) $\text{A}$ or $\text{G}$.

  • Anticodon First Base ($5'$ end) $\text{I}$ (Inosine, a modified purine) pairs with Codon Third Base ($3'$ end) $\text{A}$, $\text{U}$, or $\text{C}$.

• Universality and Evolutionary Exceptions (Table 15.3):

  • The genetic code is nearly universal across all domains of life, indicating a common evolutionary origin, but notable exceptions exist in organellar genomes and specific unicellular organisms:

  • Bacterial Genome (Mycoplasma capricolum): $\text{UGA}$ encodes Tryptophan ($\text{Trp}$) rather than Stop.

  • Human Mitochondrial DNA: $\text{UGA}$ encodes $\text{Trp}$ (universal: Stop); $\text{AUA}$ encodes Methionine ($\text{Met}$) (universal: $\text{Ile}$); $\text{AGA}$ and $\text{AGG}$ serve as Stop codons (universal: $\text{Arg}$).

  • Yeast Mitochondrial DNA: $\text{UGA}$ encodes $\text{Trp}$ (universal: Stop).

  • Trypanosome Mitochondrial DNA: $\text{UGA}$ encodes $\text{Trp}$ (universal: Stop).

  • Plant Mitochondrial DNA: $\text{CGG}$ encodes $\text{Trp}$ (universal: $\text{Arg}$).

  • Nuclear DNA of Ciliates (Tetrahymena): $\text{UAA}$ encodes Glutamine ($\text{Gln}$) (universal: Stop).

  • Nuclear DNA of Ciliates (Paramecium): $\text{UAG}$ encodes $\text{Gln}$ (universal: Stop).

The Mechanism of Translation: Initiation, Elongation, and Termination

• Stage 1: Binding of Amino Acids to Transfer RNAs (tRNA Charging):

  • All mature tRNAs terminate at their $3'$ acceptor stem with the invariant single-stranded trinucleotide sequence $5'\text{-CCA-}3'$.

  • The carboxyl group of the specific amino acid is covalently linked to the $2'$ or $3'$ hydroxyl group of the terminal adenosine ribose unit.

  • Aminoacyl-tRNA Synthetases:

  • Exactly 20 distinct aminoacyl-tRNA synthetase enzymes exist in each cell—one specific enzyme for each of the 20 standard amino acids.

  • Each synthetase recognizes both a specific amino acid and all corresponding isoaccepting tRNAs.

  • Nucleotide positions in the anticodon loop, acceptor stem, DHU loop, and extra arm provide recognition determinants that distinguish individual tRNAs.

• Two-Step Chemical Mechanism of Charging:

  • Step 1: The amino acid reacts with ATP to produce an enzyme-bound intermediate, aminoacyl-AMP, releasing inorganic pyrophosphate ($\text{PP}_i$):     $\text{Amino Acid} + \text{ATP} \rightarrow \text{Aminoacyl-AMP} + \text{PP}_i$

  • Step 2: The activated aminoacyl residue is transferred to the $3'\text{-OH}$ of the cognate tRNA, releasing free AMP:     $\text{Aminoacyl-AMP} + \text{tRNA} \rightarrow \text{Aminoacyl-tRNA} + \text{AMP}$

  • Overall Net Reaction:     $\text{Amino Acid} + \text{tRNA} + \text{ATP} \rightarrow \text{Aminoacyl-tRNA} + \text{AMP} + \text{PP}_i$

• Stage 2: Translation Initiation in Bacteria:

  • The bacterial ribosome consists of a small $30\text{S}$ subunit and a large $50\text{S}$ subunit, which associate to form the intact $70\text{S}$ ribosome.

  • Sequence of Initiation Events:

  1. Initiation Factor 3 (IF-3) binds to the $30\text{S}$ small subunit, physically blocking the premature association of the $50\text{S}$ large subunit.

  2. The $30\text{S}$ subunit binds to the mRNA via the Shine–Dalgarno consensus sequence ($5'\text{-UAAGGAGGU-}3'$), located approximately 8 nucleotides upstream of the $\text{AUG}$ initiation codon. This sequence base-pairs directly with a complementary sequence located at the $3'$ end of the 16S rRNA ($3'\text{-AUUCCUCCA-}5'$).

  3. The initiator tRNA charged with N-formylmethionine (designated $\text{fMet-tRNA}^{\text{fMet}}$) forms a ternary complex with Initiation Factor 2 (IF-2) and GTP.

  4. The $\text{fMet-tRNA}^{\text{fMet}}\text{-IF-2-GTP}$ complex enters the P site and base-pairs with the $\text{AUG}$ start codon. Initiation Factor 1 (IF-1) binds simultaneously to stabilize the $30\text{S}$ complex.

  5. The resulting assembly is the $30\text{S}$ initiation complex.

  6. Hydrolysis of GTP to GDP and inorganic phosphate ($\text{P}_i$) induces conformational changes causing IF-1, IF-2, and IF-3 to dissociate.

  7. The $50\text{S}$ large ribosomal subunit joins the $30\text{S}$ subunit to create the intact $70\text{S}$ initiation complex, positioning $\text{fMet-tRNA}^{\text{fMet}}$ directly in the peptidyl (P) site.

• Translation Initiation in Eukaryotes:

  • Eukaryotes utilize an unformylated initiator methionine attached to a unique initiator tRNA ($\text{tRNA}_i^{\text{Met}}$).

  • The $40\text{S}$ small ribosomal subunit associates with multiple eukaryotic initiation factors (eIFs) and the charged $\text{Met-tRNA}_i^{\text{Met}}$.

  • The small subunit complex recognizes the $5'$ cap structure of the mRNA and migrates downstream along the $5'$ untranslated region ($5'\text{ UTR}$) scanning for the start codon.

  • The start codon is located within the Kozak consensus sequence: $5'\text{-ACCAUGG-}3'$.

  • mRNA Circularization: Cap-binding proteins associated with the $5'$ 7-methylguanosine cap interact directly with poly(A)-binding proteins bound to the $3'$ poly(A) tail, forming a closed-loop mRNA structure that enhances translational efficiency and subunit recruitment.

• Stage 3: Translation Elongation:

  • An active ribosome possesses three operational binding sites spanning both subunits:

  • A Site (Aminoacyl Site): Accepts incoming aminoacyl-tRNAs matching the displayed codon.

  • P Site (Peptidyl Site): Retains the tRNA covalently attached to the growing nascent polypeptide chain.

  • E Site (Exit Site): Holds the deacylated, uncharged tRNA prior to cytoplasmic release.

  • Elongation Cycle Mechanism:

  • Step 1: Codon Recognition and A-site Entry: An incoming charged aminoacyl-tRNA complexes with Elongation Factor Tu (EF-Tu) and GTP. This ternary complex binds to the mRNA codon positioned in the vacant A site. GTP is hydrolyzed to GDP, releasing EF-Tu-GDP. Elongation Factor Ts (EF-Ts) regenerates active EF-Tu-GTP.

  • Step 2: Peptide Bond Formation: A peptide bond is synthesized between the amino acid in the A site and the carboxyl terminus of the peptide chain in the P site. This reaction is catalyzed by the peptidyl transferase center, a catalytic ribozyme constituted by the 23S rRNA of the $50\text{S}$ large ribosomal subunit. The polypeptide is transferred entirely to the tRNA residing in the A site.

  • Step 3: Translocation: Elongation Factor G (EF-G) and GTP bind the ribosome. GTP hydrolysis drives a conformational movement shifting the ribosome precisely three nucleotides ($1\text{ codon}$) downstream along the mRNA in the $5' \rightarrow 3'$ direction:

    • The deacylated tRNA moves from the P site into the E site, where it dissociates into the cytoplasm.

    • The peptidyl-tRNA bearing the growing polypeptide shifts from the A site into the P site.

    • The A site is vacated and ready to receive the next charged tRNA.

• Stage 4: Translation Termination:

  • When a termination codon ($\text{UAA}$, $\text{UAG}$, or $\text{UGA}$) translocates into the ribosomal A site, no cellular tRNA carries a complementary anticodon.

  • Specialized protein Release Factors (RFs) enter the A site:

  • RF-1: Recognizes termination codons $\text{UAA}$ and $\text{UAG}$.

  • RF-2: Recognizes termination codons $\text{UAA}$ and $\text{UGA}$.

  • RF-3: Binds GTP and associates with the ribosome.

  • The binding of RF-1 or RF-2 alters peptidyl transferase activity, triggering the cleavage of the ester bond connecting the completed polypeptide to the tRNA in the P site.

  • The synthesized polypeptide chain is released from the ribosome.

  • Hydrolysis of GTP associated with RF-3 stimulates the complete disassembly of the translation complex: the deacylated tRNA, mRNA, release factors, and ribosomal subunits dissociate.

• Comprehensive Summary of Bacterial Protein Synthesis Components (Table 15.4):

  • tRNA Charging:

  • Amino acids: Building blocks of proteins.

  • tRNAs: Deliver amino acids to ribosomes.

  • Aminoacyl-tRNA synthetases: Attach amino acids to tRNAs.

  • ATP: Provides energy for binding amino acid to tRNA.

  • Initiation:

  • mRNA: Carries coding instructions.

  • $\text{fMet-tRNA}^{\text{fMet}}$: Provides first amino acid in peptide.

  • $30\text{S}$ ribosomal subunit: Attaches to mRNA.

  • $50\text{S}$ ribosomal subunit: Stabilizes tRNAs and amino acids.

  • Initiation factor 1 (IF-1): Enhances dissociation of large and small subunits of ribosome.

  • Initiation factor 2 (IF-2): Binds GTP; delivers $\text{fMet-tRNA}^{\text{fMet}}$ to initiation codon.

  • Initiation factor 3 (IF-3): Binds to $30\text{S}$ subunit and prevents association with $50\text{S}$ subunit.

  • Elongation:

  • $70\text{S}$ initiation complex: Functional ribosome with A, P, and E sites where protein synthesis takes place.

  • Charged tRNAs: Bring amino acids to ribosome and assemble them in order specified by mRNA.

  • Elongation factor Tu (EF-Tu): Binds GTP and charged tRNA; delivers charged tRNA to A site.

  • Elongation factor Ts (EF-Ts): Generates active elongation factor Tu.

  • Elongation factor G (EF-G): Stimulates movement of ribosome to next codon.

  • GTP: Provides chemical energy.

  • Peptidyl transferase center: Creates peptide bond between amino acids in A site and P site.

  • Termination:

  • Release factors 1, 2, and 3: Bind to ribosome when stop codon is reached and terminate translation.

Ribosome Structure, Polyribosomes, and Translational Quality Control

• Three-Dimensional Ribosome Architecture:

  • X-ray crystallography reveals that ribosomal RNAs constitute the structural and catalytic framework of the ribosome, while ribosomal proteins occupy peripheral surfaces.

  • Bacterial 70S Ribosome Components:

  • Large Subunit ($50\text{S}$): Composed of 23S rRNA ($\sim 2900\text{ nucleotides}$), 5S rRNA ($\sim 120\text{ nucleotides}$), and 31 distinct ribosomal proteins.

  • Small Subunit ($30\text{S}$): Composed of 16S rRNA ($\sim 1500\text{ nucleotides}$) and 21 ribosomal proteins.

  • The active centers (decoding center on the $30\text{S}$ subunit; peptidyl transferase center on the $50\text{S}$ subunit) consist entirely of rRNA, demonstrating that the ribosome functions intrinsically as a ribozyme.

• Polyribosomes (Polysomes) and Spatial Regulation:

  • A single mRNA molecule can be translated simultaneously by multiple consecutive ribosomes moving in the $5' \rightarrow 3'$ direction. This complex is designated a polyribosome or polysome.

  • As each ribosome translocates along the message, its nascent polypeptide chain progressively elongates.

  • Prokaryotes vs. Eukaryotes:

  • Prokaryotes: Lacking a nuclear envelope, transcription and translation are coupled—ribosomes attach to the $5'$ end of the nascent mRNA and initiate translation while RNA polymerase is still synthesizing the $3'$ region of the transcript.

  • Eukaryotes: Transcription and translation are uncoupled and compartmentalized. Transcription and pre-mRNA processing occur exclusively inside the nucleus; mature mRNAs are subsequently exported through nuclear pores to the cytoplasm for translation.

• Messenger RNA Surveillance and Stalled Ribosome Rescue:

  • Nonsense-Mediated mRNA Decay (NMD):

  • Surveillance mechanisms detect aberrant mRNAs containing premature termination codons resulting from nonsense mutations or transcription errors.

  • Such defective transcripts are degraded prior to translation to prevent the synthesis of potentially toxic truncated proteins.

  • Bacterial tmRNA and Stalled Ribosomes:

  • Ribosomes stall when translating damaged, truncated mRNAs that lack a stop codon before the $3'$ physical terminus. Stalled ribosomes cannot terminate, cannot release the nascent peptide, and cannot dissociate, thereby depleting the cellular pool of active ribosomes and reducing global translation.

  • Bacterial cells resolve this crisis using transfer-messenger RNA (tmRNA), a bifunctional hybrid molecule that functions both as a tRNA and as an mRNA:

    1. tmRNA is charged with an Alanine residue ($\text{Ala}$) by alanyl-tRNA synthetase.

    2. The alanyl-tmRNA binds into the empty A site of the stalled ribosome.

    3. Peptidyl transferase links the alanine to the stalled polypeptide chain.

    4. The ribosome resumes translation, shifting off the defective transcript and onto the mRNA domain of the tmRNA.

    5. The tmRNA template directs the synthesis of a specific 10-amino-acid degradation tag ending in an in-frame stop codon ($\text{UAG}$).

    6. Standard release factors enter the A site, cleaving and releasing the tagged polypeptide.

    7. The tag targets the truncated protein for destruction by cellular proteases, and the recycled ribosomal subunits re-enter the functional pool.

• Posttranslational Modifications and Antibiotics:

  • Newly translated polypeptides undergo folding into native tertiary structures, frequently mediated by molecular chaperones that prevent nonfunctional aggregation.

  • Posttranslational modifications may include chemical group addition (phosphorylation, methylation, acetylation, glycosylation), prosthetic group attachment, proteolytic cleavage of signal sequences or inhibitory domains, and disulfide bond formation.

  • Many commercial antibiotics (such as tetracyclines, aminoglycosides, chloramphenicol, and erythromycin) selectively inhibit bacterial translation by binding to unique structural pockets in prokaryotic $70\text{S}$ ribosomes, sparing eukaryotic $80\text{S}$ cytoplasmic ribosomes.

Concept Checks & Problem-Solving Scenarios

• Biochemical Pathway Ordering (Concept Check 1):

  • Scenario: Three auxotrophic mutant strains are tested for growth on minimal medium supplemented with compounds A, B, or C:

  • Mutant 103 grows when supplemented with A, B, or C.

  • Mutant 106 grows when supplemented with A or C, but not B.

  • Mutant 102 grows only when supplemented with C.

  • Deduction:

  • An intermediate that rescues the greatest number of different mutants must act latest in the pathway.

  • Compound C rescues all three mutant strains (103, 106, 102), proving it is the final compound.

  • Compound A rescues 103 and 106, indicating it is downstream of compound B but upstream of C.

  • Compound B rescues only 103, indicating it acts earliest among the supplemented intermediates.

  • Mutant 103 is blocked prior to compound B; mutant 106 is blocked between B and A; mutant 102 is blocked between A and C.

  • Pathway Sequence: $\text{Precursor} \rightarrow \text{B} \rightarrow \text{A} \rightarrow \text{C}$

• Primary Determinants of Protein Conformation (Concept Check 2):

  • Question: What primarily determines the secondary and tertiary structures of a protein?

  • Answer: The primary structure (the specific linear sequence of amino acids) dictates all thermodynamic folding pathways and interactions that produce the secondary and tertiary structures.

• Mechanisms of Wobble Degeneracy (Concept Check 3):

  • Question: Through wobble, a single _____ can pair with more than one _____?

  • Answer: A single anticodon on a tRNA molecule can pair with more than one codon on an mRNA molecule.

• tRNA Molecular Topography (Concept Check 4):

  • Question: Amino acids bind to which specific part of the transfer RNA?

  • Answer: The $3'$ end (specifically to the terminal adenosine residue of the $5'\text{-CCA-}3'$ acceptor stem).