Control of Protein Synthesis: Transcription, Translation, Posttranscriptional Processing

15.3 Eukaryotic Transcription

  • Prokaryotes vs. eukaryotes

    • Eukaryotes have a membrane-bound nucleus and organelles, requiring export and protection of mRNA before translation.
    • Eukaryotes employ three RNA polymerases, each transcribing a different subset of genes; transcription and translation are spatially separated in time.
    • Eukaryotic mRNAs are usually monogenic (specify a single protein).
    • In contrast, prokaryotes can have transcription and translation happening concurrently in the same cellular compartment (no nucleus).
    • The unification of transcription, translation, and mRNA degradation in prokaryotes is possible because these processes can occur in the same 5′ to 3′ direction and within the same compartment.
    • Visual aid: multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate, enabling rapid high protein concentrations.

  • Initiation of transcription in eukaryotes

    • Eukaryotic RNA polymerases require several transcription factors to bind the promoter and recruit the polymerase, unlike prokaryotic RNA polymerase that can bind directly to a DNA template.
    • The transcription initiation complex (preinitiation complex, PIC) is formed with promoter-bound transcription factors and RNA polymerase II.

  • The Three Eukaryotic RNA Polymerases

    • RNA polymerase I (Pol I)
    • Location: nucleolus.
    • Product: All rRNAs except 5S rRNA.
    • α-Amanitin sensitivity: Insensitive.
    • RNA polymerase II (Pol II)
    • Location: nucleus.
    • Product: All protein-coding nuclear pre-mRNAs.
    • α-Amanitin sensitivity: Extremely sensitive.
    • RNA polymerase III (Pol III)
    • Location: nucleus.
    • Product: 5S rRNA, tRNAs, and small nuclear RNAs (snRNAs).
    • α-Amanitin sensitivity: Moderately sensitive.
    • Practical takeaway: α-Amanitin sensitivity helps identify which polymerase transcribes a gene; Pol II transcribes the majority of genes in eukaryotes.

  • Table 15.1 (summary) – RNA polymerases, locations, products, and α-amanitin sensitivities

    • Pol I: Nucleolus; All rRNAs except 5S; Insensitive.
    • Pol II: Nucleus; All protein-coding nuclear pre-mRNAs; Extremely sensitive.
    • Pol III: Nucleus; 5S rRNA, tRNAs, and snRNAs; Moderately sensitive.

  • Promoters of RNA Polymerase II

    • Eukaryotic promoters are larger and more complex than prokaryotic promoters but share the feature of a TATA box.
    • TATA box location: around −30-30 relative to the transcription start site (+1).
    • Typical TATA box sequence (mouse thymidine kinase gene): extTATAAAAext{TATAAAA} (read 5′→3′ on the non-template strand).
    • Rationale for AT-rich TATA box: low stability of A–T bonds facilitates local unwinding of DNA for transcription initiation.
    • Additional promoter elements upstream include:
    • CAAT box at approximately −80-80.
    • GC-rich boxes (e.g., GGCGGGCG).
    • Octamer boxes (e.g., ATTTGCATATTTGCAT).
    • These elements bind cellular transcription factors to increase transcription initiation efficiency and are often found in genes that are constantly expressed.

  • Structure of a Pol II promoter and transcription factors

    • Promoters recruit basal transcription factors (TFII family; TFIIA–TFIJI) to form the PIC and recruit Pol II.
    • Enhancers and silencers modulate transcription frequency but are not strictly required for transcription to proceed.
    • The assembly of the PIC stabilizes and helps recruit RNA polymerase II.
    • Similar but simpler factor sets recruit Pol I and Pol III to their respective templates.

  • Evolution of promoters

    • Promoter sequences can evolve rapidly, sometimes even faster than protein-coding genes, influencing gene expression levels.
    • Defining exact promoter boundaries is challenging; promoters can be within genes or far upstream or downstream of the regulated gene.
    • Experimental work on human core promoter sequences shows fast promoter evolution relative to protein-coding genes, highlighting promoter evolution as a key regulatory layer in evolution.

  • Promoter structures for RNA Polymerases I and III

    • Pol I promoters:
    • Contain two GC-rich promoter sequences in the −45-45 to +20+20 region; these are sufficient to initiate transcription.
    • Additional upstream sequences from −180-180 to −105-105 further enhance initiation.
    • Pol III promoters:
    • Can be upstream promoters or promoters located within the genes themselves.

  • Eukaryotic elongation and termination

    • After PIC formation, Pol II elongation proceeds similarly to prokaryotic elongation (5′ to 3′ direction) but within a chromatin context.
    • Chromatin packaging: DNA is wound around histones to form nucleosomes (DNA–histone complexes) that must be moved or remodeled for transcription.
    • FACT complex (facilitates chromatin transcription) helps displace histones ahead of the polymerase and reassemble nucleosomes behind it.
    • Termination differences:
    • Pol II often transcribes 1,000–2,000 nucleotides beyond the gene end; this tail is processed later into mature mRNA.
    • Pol I termination involves an 18-nucleotide termination sequence recognized by a termination protein.
    • Pol III termination involves a hairpin structure in the transcript, analogous to rho-independent termination in prokaryotes.

  • Overview: RNA processing in eukaryotes (lead-in to 15.4)

  • Important concept: pre-mRNA processing is essential to produce mature mRNA with a long half-life and proper translation signals.

15.4 RNA Processing in Eukaryotes

  • Overview of processing steps after transcription

    • Eukaryotic pre-mRNAs undergo extensive processing before translation.
    • Processing steps significantly extend mRNA half-life compared with prokaryotic mRNA (eukaryotic mRNAs last for hours; bacterial mRNA half-life is only ~5 seconds).
    • Pre-mRNAs are coated with stabilizing proteins to protect from degradation during processing and export.
    • Three core processing steps: 5′ capping, 3′ poly-A tail addition, and removal of introns (splicing).
    • In some rare cases, RNA editing modifies the transcript after transcription.

  • RNA editing in Trypanosomes (a notable exception)

    • Trypanosoma brucei pre-mRNAs often require RNA editing to generate functional transcripts, especially mitochondrial genes missing nucleotides (notably U).
    • Guide RNAs (gRNAs) (~40–80 nt) interact with pre-mRNA and template insertion/deletion occurs, often inserting Us.
    • The editing reaction is RNA-guided and RNA-catalyzed, not protein-catalyzed, in this system.
    • RNA editing is not exclusive to trypanosomes; similar editing occurs in plant mitochondria and in mammals (rats, rabbits, humans).
    • Evolutionary speculation: RNA-based catalysis in mitochondria may reflect ancient biochemistry before protein enzymes; edits can vary with cellular conditions.
    • Practical implication: RNA editing represents an extra layer of post-transcriptional control and adaptability.

  • 5′ Capping

    • During transcription, a 7-methylguanosine cap is added to the 5′ end via a phosphate linkage.
    • The cap protects the nascent mRNA from degradation and is recognized by cap-binding proteins to promote translation initiation.

  • 3′ Poly-A Tail

    • Cleavage occurs at an AAUAAA consensus sequence followed by a GU-rich sequence.
    • Poly-A polymerase adds ~200 adenine residues to the 3′ end (the poly-A tail).
    • The tail protects mRNA from degradation and promotes export to the cytoplasm.

  • Pre-mRNA Splicing

    • Introns: noncoding intervening sequences; Exons: coding sequences that will be expressed.
    • Introns are removed and exons joined by spliceosomes, complexes of proteins and snRNAs (small nuclear RNAs).
    • Spliceosomes recognize splice sites at the 5′ and 3′ ends of introns, ensuring precise exon joining.
    • Consequences of splicing errors include misfolded proteins and links to cancer and other diseases.
    • Many genes contain multiple introns; the splicing process is essential to produce a single translatable mRNA per gene.

  • Processing of tRNAs and rRNAs

    • tRNAs and rRNAs are structural RNAs involved in translation; they are transcribed and processed separately from mRNA.
    • Pre-rRNAs are processed in the nucleolus and assembled into ribosomes; pre-tRNAs are processed in the nucleus and exported to the cytoplasm.
    • Methylation of certain nucleotides occurs in pre-rRNAs and pre-tRNAs, aiding stability.
    • Mature rRNAs comprise roughly 50% of a ribosome; tRNAs serve as adapters that deliver amino acids to the ribosome.
    • tRNA structure includes an anticodon that base-pairs with mRNA codons and an amino acid attachment site on the 3′ end.
    • The anticodon triplet interacts with codons following the genetic code; example illustration provided: anticodon GAU pairs with codon CUA to insert leucine (example from the text).

  • Summary of mature RNA products

    • Mature mRNAs: capped, polyadenylated, intronless (exon-only) in coding regions, exported to cytoplasm for translation.
    • Pre-tRNAs and pre-rRNAs processed to mature rRNA and tRNA components of the ribosome and translation machinery.

15.5 Ribosomes and Protein Synthesis

  • Overview: protein synthesis is energetically costly and essential for cellular function; ribosomes, tRNAs, and various factors drive translation.

  • Numerical and structural basics

    • Bacterial ribosome: 70S total; subunits 50S (large) and 30S (small). Svedberg units (S) are sedimentation coefficients; not strictly additive: 50S+30S ≈ 70S.
    • Eukaryotic ribosome: 80S total; subunits 60S (large) and 40S (small).
    • Mitochondria and chloroplasts have ribosomes that resemble bacterial ribosomes in size and drug sensitivities.
    • A single mRNA can be translated by multiple ribosomes simultaneously, forming a polysome.

  • Translation components

    • mRNA template, ribosomes (rRNA + proteins), tRNAs, and various enzymatic factors.
    • tRNAs: 40–60 types in cytoplasm (species-dependent); adapters that bring the correct amino acid to the ribosome by recognizing codons on mRNA and aminoacyl tRNA synthetases.
    • Aminoacyl tRNA synthetases: charge tRNAs with their correct amino acids; at least one synthetase per amino acid; mechanism involves ATP-dependent activation of the amino acid to aminoacyl-AMP followed by transfer to tRNA.

  • The genetic code and codon recognition

    • 64 possible mRNA codons; 61 codons specify amino acids; 3 codons are stop signals.
    • Initiation codon AUG also encodes methionine; in bacteria, formylated methionine (fMet) is used to initiate translation; in many bacteria, initiator tRNAfMet participates in initiation.
    • Each tRNA contains an anticodon that base-pairs with its codon, delivering the corresponding amino acid.
    • Example (as described in the transcript): a CUA codon on mRNA would pair with a complementary anticodon GAU on a tRNA delivering leucine.

  • Initiation of translation

    • Prokaryotes (E. coli-like): initiation involves the 30S subunit, mRNA, three initiation factors (IF-1, IF-2, IF-3), and a special initiator tRNA (tRNAfMet).
    • The initiator tRNA (fMet-tRNAfMet) binds the start codon (AUG) and, with IF-2, forms the initiation complex before the 50S subunit joins.
    • The Shine-Dalgarno sequence (e.g., AGGAGG) is located upstream of the start codon and base-pairs with the 16S rRNA to position the ribosome correctly on the mRNA.
    • Energy input for initiation and translocation is provided by GTP hydrolysis: GTP acts as an energy source during initiation and translocation.
    • Eukaryotes: initiation involves the 40S subunit, mRNA, initiation factors, and GTP/ATP; Met-tRNAi is used (not formylated).
    • Eukaryotic initiation relies on recognition of the 5′ cap by cap-binding proteins (CBP) and initiation factors to recruit the ribosome to the 5′ end of mRNA.
    • Start-site selection in eukaryotes is guided by Kozak’s rules: the consensus around AUG is 5′ext−gccRccAUGGext−3′5' ext{-}gccRccAUGG ext{-}3' where R is a purine (A or G); closer matches increase initiation efficiency.
    • After AUG recognition, initiation factors dissociate and the 60S subunit joins to complete the 80S initiation complex in eukaryotes.

  • Elongation, codon recognition, and peptide bond formation

    • Elongation basics are shared: correct tRNA entry into the A site, peptide bond formation between A-site and P-site amino acids, translocation to the next codon, and E-site tRNA release.
    • The A site binds aminoacyl-tRNA; the P site holds the growing polypeptide attached to tRNA; the E site releases spent tRNA.
    • Each codon is read in a single-base step, with three-nucleotide steps advancing the ribosome along the mRNA. Elongation factors hydrolyze GTP to power these steps.
    • Peptidyl transferase, an RNA-based catalytic activity within the 50S subunit, forms the peptide bond between the amino acid on the A-site tRNA and the growing polypeptide attached to the P-site tRNA.
    • The energy for peptide bond formation is derived from GTP hydrolysis driven by an elongation factor.
    • Kinetics note: in E. coli, translation of a protein of about 200 amino acids can occur in roughly 10exts10 ext{ s} (≈ 0.05 s per amino acid).

  • Termination of translation

    • Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site.
    • Release factors recognize the stop codons and trigger hydrolysis by peptidyl transferase to release the polypeptide and add a water molecule to the C-terminus of the peptidyl-tRNA, freeing the completed protein.
    • The ribosomal subunits dissociate and recycle for a new round of initiation.

  • Antibiotics that target bacterial translation

    • Tetracycline blocks tRNA entry at the A site; thus, it directly affects tRNA binding to the ribosome.
    • Chloramphenicol inhibits peptidyl transferase, directly affecting peptide bond formation.
    • Practical implication: these drugs selectively inhibit bacterial protein synthesis by targeting bacterial ribosomal components.

  • Protein folding, modification, and targeting

    • Co-translational and post-translational processes tailor a newly synthesized protein for its final cellular location and function.
    • Signal sequences direct proteins to specific organelles (e.g., mitochondria, chloroplasts); these signals are often cleaved after import.
    • Chaperones assist in proper folding and prevent aggregation; misfolding can occur under stress conditions (temp/pH).

  • Cross-cutting themes and connections

    • The flow from DNA to functional protein involves multiple regulated steps: transcription initiation, RNA processing, nuclear export, translation, folding, and targeting.
    • Regulation occurs at multiple levels: promoter architecture, transcription factors, chromatin remodeling (FACT), RNA processing choices, splicing, and translation initiation controls (Kozak context, cap recognition).
    • The central dogma concept is extended by RNA editing (notably in mitochondria of some organisms), RNA splicing complexity, and post-translational modifications that expand functional outcomes beyond the primary sequence.

  • Key terms to remember

    • Nucleosome, FACT, promoter elements (TATA box, CAAT box, GC boxes, octamer boxes), preinitiation complex, transcription factors (TFIIA–TFIJI), Spliceosome, snRNA, 5′ cap, poly-A tail, AAUAAA signal, poly-A polymerase, Shine-Dalgarno sequence, Kozak consensus, Met-tRNAi, fMet-tRNAfMet, tRNA synthetases, A site, P site, E site, peptidyl transferase, release factors, stop codons (UAA, UAG, UGA).

  • Illustrative and broader implications

    • Splicing errors are implicated in various diseases, including cancers, highlighting the importance of precise RNA processing.
    • Promoter evolution contributes to species- and tissue-specific gene expression profiles and may outpace coding sequence evolution.
    • RNA editing and the use of guide RNAs reflect ancient regulatory mechanisms that can respond to cellular conditions.
    • Antibiotics targeting translation underscore the clinical relevance of basic molecular biology to medicine.

  • Quick practice prompts from the material

    • Identify the three RNA polymerases and their primary products and sensitivities to α-amanitin.
    • Explain how the Kozak sequence influences initiation in eukaryotes and contrast with the Shine-Dalgarno mechanism in prokaryotes.
    • Describe the role of the FACT complex in transcription elongation through chromatin.
    • Compare the ribosomal subunit composition and overall translation machinery between bacteria and eukaryotes.
    • Outline the steps of mRNA maturation (capping, tailing, splicing) and the purpose of each.

  • Note about sources and further exploration

    • For a visual overview of prokaryotic transcription dynamics, refer to the BioStudio animation linked in the text: http://openstaxcollege.org/l/transcription2
    • Splicing and RNA processing animations and interactive resources are available via OpenStax references (e.g., RNA splicing website).

  • Equations and quantitative details (for quick reference)

    • Directionality of transcription: extDNAto5′<br/>ightarrow3′extandmRNAissynthesized5′<br/>ightarrow3′ext{DNA to }5'<br /> ightarrow 3' ext{ and mRNA is synthesized } 5'<br /> ightarrow 3'
    • Promoter coordinates: TATA box around −30-30 relative to +1 start site.
    • Distance for Pol I promoter enhancement: −180extto−105-180 ext{ to } -105 upstream region.
    • Elongation beyond gene end for Pol II: 1,000extto2,0001{,}000 ext{ to }2{,}000 nucleotides beyond gene end.
    • Poly(A) tail length: ext{approximately } 200200 adenine residues.
    • Ribosome sizes: 70Sext(bacteria),80Sext(eukaryotes)70S ext{ (bacteria)}, 80S ext{ (eukaryotes)}; subunits: 30S/50S30S/50S and 40S/60S40S/60S respectively.
    • Translation rate: about 0.05extsperaminoacid0.05 ext{ s per amino acid}; a 200-amino-acid protein in roughly 10exts10 ext{ s}.
    • Codons: 64 total; 61 sense codons; 3 stop codons; start codon AUG.
    • Kozak consensus: 5′−gccRccAUGG−3′5'-gccRccAUGG-3' with R = purine.

  • End of notes