Translation

Translation in Bacteria

  • Overview of Translation

    • Directed synthesis of polypeptides based on the sequence of nucleotides in mRNA.

    • Translation occurs in the ribosome, which serves as the site of the process.

    • The synthesis direction is from -terminal (N-terminal) to C-terminal.

  • Coupled Transcription and Translation in Bacteria and Archaea

    • Coupling allows for simultaneous processes where polyribosomes are formed.

    • Polyribosomes consist of a complex of mRNA with several ribosomes translating the mRNA simultaneously.

Transfer RNA (tRNA)

  • Structure of tRNA

    • Exhibits a tertiary structure due to base pairing within the molecule.

    • Acceptor Stem:

    • The 3’ end features a conserved CCA sequence.

    • Anticodon:

    • Present on the anticodon arm and complementary to the mRNA codon.

Amino Acid Activation

  • Mechanism

    • Attachment of an amino acid to its corresponding tRNA.

    • Catalyzed by aminoacyl-tRNA synthetases.

    • At least 20 different synthetases exist, each specific for a single amino acid and corresponding tRNAs.

The Ribosome

  • Structure

    • Bacterial ribosomes are 70S and consist of two subunits: 30S and 50S.

    • Features two functional domains:

    • Translational Domain: Involved in translation.

    • Exit Domain: Facilitates the exit of the tRNA.

  • Role of rRNA

    • Carl Woese's Research: Utilized rRNA sequences to construct the tree of life, aiding in determining evolutionary distance among species.

    • 16S rRNA:

    • Serves as a molecular clock for bacterial species.

    • Important for binding to the ribosomal binding site (RBS) on mRNA, which is complementary to the sequence at the 3' end of 16S rRNA.

Initiation of Protein Synthesis

  • Bacterial Initiation tRNA

    • Bacteria utilize N-formylmethionine-tRNA as the initiator tRNA.

  • Process of Initiation

    • Begins when the initiator codon binds to the Shine-Dalgarno sequence of mRNA, aligning with 16S rRNA.

    • Involves initiation factors (IF-1 and IF-2) to form the initiation complex:

    • IF-3 prevents premature binding of the 50S subunit to the 30S.

    • The binding is guided by GTP; as the 50S subunit binds, GTP is hydrolyzed, and IFs are released, forming the 70S initiation complex.

Elongation of the Polypeptide Chain

  • Phases of Elongation

    • Three Main Steps:

    1. Binding of the amino acid to tRNA.

    2. Transpeptidation Reaction: Catalyzed by a 23S rRNA ribozyme, where the amino group of the A site amino acid reacts with the carboxyl group of the C-terminal amino acid on the P site tRNA.

    3. Translocation: Involves elongation factors (EFs).

      • EF-Tu: Binds GTP and aminoacyl-tRNA, facilitating its transport to the A site. Hydrolysis of GTP occurs during this step.

      • EF-G: Binds GTP, using energy provided by GTP hydrolysis to move the ribosome down the mRNA to the next codon, displacing the empty tRNA to the E site.

  • Binding Sites of Ribosome

    • P Site (Peptidyl Site): Binds initiator tRNA or tRNA with the growing polypeptide.

    • A Site (Aminoacyl Site): Accepts the incoming aminoacyl-tRNA.

    • E Site (Exit Site): Briefly binds empty tRNA before its release from the ribosome.

Termination of Protein Synthesis

  • Codon Recognition

    • Takes place at any of the three stop codons: UAA, UAG, UGA.

    • Release factors (RFs) recognize these codons.

    • Three RFs function in prokaryotes, while only one operates in eukaryotes.

    • GTP hydrolysis is necessary for the termination process to complete successfully.

Polyribosomes and Transcription/Translation Coupling

  • Functionality in Prokaryotes

    • Multiple ribosomes can bind simultaneously to each cistron in a polycistronic mRNA.

    • As transcription proceeds, ribosomes can engage with the mRNA at the 5′ end prior to the completion of transcription.

Protein Maturation and Secretion

  • Protein Splicing

    • Involves the removal of certain polypeptide portions (inteins) while leaving others (exteins) which remain functional in the protein structure.

    • Protein function is heavily influenced by the protein's three-dimensional shape established post-translationally through folding and proper localization.

  • Role of Molecular Chaperones

    • Chaperones such as heat-shock proteins aid in the folding of proteins and protecting from thermal damage.

  • Mechanisms of Protein Translocation and Secretion

    • Translocation: Movement from the cytoplasm to either the plasma membrane or periplasmic space (Sec and Tat systems).

    • Secretion: Passage of proteins from the cytoplasm into the external environment, often involving sequenced processes across membranes.

    • Types II, V, and IX systems accomplish secretion in two steps, while Types I, III, IV, VI, and VII are one-step secretion systems.

Antibiotics Targeting Translation and Transcription

  • Translation Inhibitors:

    • Streptomycin: Inhibits 70S ribosome formation.

    • Tetracycline: Binds to the 30S subunit, inhibiting translation.

    • Chloramphenicol and Erythromycin: Bind to 23S rRNA of the 50S subunit, blocking translation.

  • Transcription Inhibitors:

    • Rifamycin B: Selectively binds to the bacterial RNA polymerase, inhibiting transcription.

    • Actinomycin D: Non-selectively binds to DNA, also inhibiting transcription.

Conclusion

  • Understanding the processes involved in the translation of genetic information into proteins is crucial for molecular biology.

    • Focus on the detailed phases, mechanisms, and underlying principles ensures a comprehensive knowledge foundation suitable for further study in biological sciences.