Translation is the final step in protein synthesis, where mRNA is decoded to produce a polypeptide. This process involves complex machinery and regulatory mechanisms to ensure accurate and efficient protein production. From initiation to termination, various factors work together to orchestrate this crucial cellular function.
Regulation of translation allows cells to fine-tune gene expression in response to environmental cues. Mechanisms like attenuation, upstream open reading frames, and RNA interference provide additional layers of control, enabling rapid adjustments to protein synthesis without altering transcription.
Translation Factors
Initiation, Elongation, and Release Factors
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- Initiation factors facilitate the assembly of the translation initiation complex
- eIF4E binds to the 5' cap of mRNA
- eIF4G acts as a scaffold protein, connecting eIF4E and eIF4A
- eIF2 delivers the initiator tRNA to the small ribosomal subunit
- Elongation factors assist in the addition of amino acids to the growing polypeptide chain
- EF-Tu brings aminoacyl-tRNAs to the A site of the ribosome
- EF-G promotes translocation of the ribosome along the mRNA
- Release factors recognize stop codons and terminate translation
- RF1 recognizes UAA and UAG stop codons
- RF2 recognizes UAA and UGA stop codons
- RF3 stimulates the release of RF1 and RF2 from the ribosome
Factor Functions and Interactions
- Initiation factors work together to form the pre-initiation complex
- eIF4F complex (composed of eIF4E, eIF4G, and eIF4A) unwinds secondary structures in the 5' UTR
- eIF3 bridges the connection between mRNA and the small ribosomal subunit
- Elongation factors cycle on and off the ribosome during each round of peptide bond formation
- EF-Tu hydrolyzes GTP upon correct codon-anticodon matching
- EF-Ts acts as a guanine nucleotide exchange factor for EF-Tu
- Release factors interact with the ribosome and mRNA to ensure proper translation termination
- RF1 and RF2 have similar structures but different stop codon specificities
- RF3 uses GTP hydrolysis to promote the dissociation of RF1 and RF2
Ribosome Binding and Polysomes
Shine-Dalgarno Sequence and Ribosome Recruitment
- Shine-Dalgarno sequence serves as a ribosome binding site in prokaryotes
- Located approximately 8-10 nucleotides upstream of the start codon
- Complementary to the 3' end of the 16S rRNA in the small ribosomal subunit
- Sequence consensus is AGGAGG in most bacteria
- Variations in the sequence can affect translation efficiency
- Eukaryotes use a different mechanism for ribosome recruitment
- Scanning model involves the ribosome moving along the 5' UTR until it encounters the start codon
Polyribosomes and Translation Efficiency
- Polyribosomes (polysomes) consist of multiple ribosomes translating a single mRNA molecule
- Increase the efficiency of protein synthesis by allowing simultaneous translation
- Can produce multiple copies of a protein from a single mRNA transcript
- Formation of polysomes depends on mRNA stability and translation initiation rate
- Longer mRNAs can accommodate more ribosomes
- Highly expressed genes often have optimized codon usage for efficient translation
- Polysome profiling techniques used to study translation efficiency
- Sucrose gradient centrifugation separates mRNAs based on the number of associated ribosomes
- Provides insights into translational regulation and mRNA stability
Translational Regulation Mechanisms
Attenuation and Upstream Open Reading Frames
- Translational attenuation modulates gene expression in response to specific signals
- Occurs in prokaryotes, often in operons involved in amino acid biosynthesis
- Leader sequence contains regulatory codons that respond to amino acid availability
- Upstream open reading frames (uORFs) regulate translation of the main coding sequence
- Located in the 5' UTR of some eukaryotic mRNAs
- Can inhibit translation of the downstream main ORF
- GCN4 in yeast (regulates amino acid biosynthesis genes) contains four uORFs
RNA Interference and Post-transcriptional Regulation
- RNA interference (RNAi) uses small RNA molecules to regulate gene expression
- Small interfering RNAs (siRNAs) derived from double-stranded RNA
- microRNAs (miRNAs) produced from endogenous hairpin-structured precursors
- RNAi mechanism involves the RNA-induced silencing complex (RISC)
- Argonaute proteins are key components of RISC
- Guide RNA directs RISC to complementary mRNA targets
- RNAi can lead to mRNA degradation or translational repression
- Perfect base-pairing often results in mRNA cleavage
- Imperfect base-pairing typically causes translational inhibition
- Applications of RNAi in research and potential therapeutic uses
- Gene function studies through targeted knockdown
- Development of RNAi-based drugs for various diseases (cancer, viral infections)