Chapter 11: Transcription of the Genetic Code: The Biosynthesis of RNA

Chapter 11: Transcription of the Genetic Code: The Biosynthesis of RNA

Chapter Outline

  1. Overview of transcription

  2. Transcription in prokaryotes

  3. Transcription regulation in prokaryotes

  4. Transcription in eukaryotes

  5. Transcription regulation in eukaryotes

  6. Noncoding RNAs

  7. Structural motifs in DNA-binding proteins

  8. Posttranscriptional RNA modifications

  9. Ribozymes

Overview of Transcription

  • Transcription is the process of synthesizing RNA from a DNA template and serves as the primary control point for gene expression and enzyme production.

    • One strand of the double-stranded DNA serves as the template for creating a complementary RNA sequence.

    • The transcription process results in all types of RNA, including mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

    • Notable differences exist between transcription in prokaryotes and eukaryotes.

Common Features of All Transcription

  1. Template Dependence: RNA is synthesized using a DNA template via the enzyme DNA-dependent RNA polymerase.

  2. Nucleotide Requirement: The synthesis requires all four ribonucleoside triphosphates (ATP, GTP, CTP, UTP) and Mg²⁺.

  3. No Primer Needed: Unlike DNA synthesis, a primer is not necessary for RNA synthesis, although a DNA template is required.

  4. Directionality: The RNA chain elongates from the 5' to the 3' end, retaining the triphosphate group at the 5' end (ppp).

  5. Base Sequence Signals: The enzyme utilizes one DNA strand as a template for RNA synthesis, with specific base sequence signals indicating where to initiate and terminate transcription.

  6. Template Unchanged: The DNA template remains unchanged following transcription.

Transcription in Prokaryotes

  • RNA Polymerase:

    • This enzyme catalyzes RNA production on a DNA template and has a multi-subunit structure consisting of α, ω, β, β′, and σ subunits.

    • Holoenzyme: This is the complete enzyme with all necessary components, including coenzymes.

    • The holoenzyme associates with specific DNA sequences to transcribe only the designated template strand.

    • The σ subunit recognizes the promoter and is released once transcription has commenced.

Template and Coding Strands in Prokaryotes

  • DNA comprises two strands:

    • Template Strand (Antisense or [–] Strand): Serves as the template for RNA synthesis and is read by RNA polymerase in the 3' to 5' direction.

    • Coding Strand (Sense or [+] Strand): Contains a sequence that corresponds directly to the RNA that is synthesized from the template.

Promoter Structure

  • Prokaryotes possess large amounts of DNA that do not contain coding sequences.

  • Promoters are essential DNA sequences that direct RNA polymerase towards the correct strand to initiate transcription.

Chain Initiation

  • This phase starts when RNA polymerase binds to the promoter, leading to DNA strand separation and binding of the first nucleotide to its complementary base.

  • Closed Complex: Formed when RNA polymerase binds to the promoter prior to initiation.

  • Open Complex: Occurs when transcription begins, with the DNA strands now accessible for synthesis.

Chain Elongation

  • Upon strand separation, a transcription bubble around 17 base pairs long progresses down the DNA.

  • RNA polymerase catalyzes the formation of phosphodiester bonds between incorporated ribonucleotides.

  • Topoisomerases relax supercoiling before and behind the transcription bubble, accommodating the flow of the RNA polymerase.

Torsional Strain of DNA

  • RNA polymerase creates torsional strain by scrunching the DNA into itself.

  • If RNA polymerase were to move bound to the template strand, there would be strain leading to supercoiling.

  • Topoisomerases mitigate this strain by addressing supercoils ahead and behind the advancing RNA polymerase.

Chain Termination Mechanisms

  1. Intrinsic Termination:

    • Involves termination sites with inverted repeats separated by a few base pairs, followed by adenine-rich regions that produce hairpin loops and weak binding areas between RNA and DNA.

    • The A-U pairs formed in this process only generate two hydrogen bonds, facilitating termination.

  2. Rho-Dependent Termination:

    • This mechanism requires the Rho protein, which binds to RNA and effectively chases the polymerase to facilitate the dissociation of the transcription machinery once it catches up.

Transcription Regulation in Prokaryotes

  • Prokaryotes coordinate transcription to ensure proteins are produced at optimal times and in correct quantities. Four primary methods of regulation include:

    1. Alternative σ Factors:

    • Production of different σ-subunits directs RNA polymerase to distinct genes, such as those responding to heat shock.

    1. Enhancers:

    • DNA sequences that increase the transcription rate by binding transcription factors.

    1. Operons:

    • Groups of genes encoding enzymes for metabolic pathways that can be regulated as a unit.

    1. Transcription Attenuation:

    • Regulation occurring post-initiation, allowing early termination if specific conditions are met.

Alternative σ Factors

  • Examples of organisms with σ factor variability include those like Bacillus subtilis virus and E. coli during heat shock responses.

Enhancers

  • Enhancers bind transcription factors to augment transcription rates.

  • Response Elements:

    • Special enhancers responding to metabolic signals.

  • Silencers:

    • Sequences that bind transcription factors to decrease transcription levels.

Operons

  • Operons consist of operator, promoter, and structural genes. They are not constantly transcribed; regulated by:

    • Inducers: Small molecules triggering protein production by detaching repressors from operator sites.

    • Repressor Proteins: Regulatory proteins that inhibit gene expression by attaching to operons, detaining RNA polymerase from binding to promoters.

Transcription Attenuation

  • Attenuation controls transcription regulation post-initiation with potential secondary structures influencing the process.

  • Ribosomes can either halt or continue synthesis depending on available resources like tryptophan, affecting hairpin loop formation in mRNA.

Transcription in Eukaryotes

  • Eukaryotic transcription is more intricate and relies on three RNA polymerases:

    • RNA Polymerase I:

    • Located in the nucleolus, synthesizes precursor for ribosomal RNAs.

    • RNA Polymerase II:

    • Found in nucleoplasm, synthesizes mRNA precursors.

    • RNA Polymerase III:

    • Also in nucleoplasm, synthesizes tRNAs, 5S rRNA precursors, and other RNA forms needed for mRNA processing and protein transportation.

Pol II Promoters

  • Promoters assist Pol II in recognizing the correct transcribed DNA. Four key elements are involved:

  1. Upstream Elements: Bases acting as enhancers or silencers that dictate transcription behavior.

  2. TATA Box: Sequence located 25 bases upstream of the initiation site, essential for RNA polymerase binding.

  3. Initiator Element: A variable sequence surrounding the transcription start site.

  4. Downstream Regulator: Rarely occurring elements located downstream of the initiation site.

Initiation of Transcription

  • Numerous proteins function as transcription regulators.

  • General transcription factors are essential across all promoters for eukaryotic transcription.

Elongation and Termination in Eukaryotes

  • Less clarity exists regarding elongation and termination in eukaryotes compared to prokaryotes.

  • Elongation Control:

    • Directly influenced by pause sites, premature termination, and antitermination.

  • Termination Process:

    • Termination initiates by ceasing RNA polymerase activity.

    • Polyadenylation:

    • Essential in determining termination, typically marked by the consensus sequence AAUAAA.

Regulation of Gene Expression in Eukaryotes

  • Activators: Substances that enhance transcription.

  • This process generally requires a mediator, which is a giant protein complex connecting promoters with transcription factors and regulatory elements.

  • Enhancers and Silencers: Sequences that help amplify or reduce transcription, respectively.

    • DNA looping facilitates the interaction of enhancers with transcription factors and polymerase.

Transcription and Nucleosomes

  • Nucleosome presence can inhibit transcription; effective transcription relies on RNA polymerase's interaction with promoters, made possible through chromatin rearrangements and nucleosome alterations.

  • Chromatin Remodeling Complexes:

    • ATP-dependent complexes that induce structural changes in nucleosomes, enabling access to genetic material.

  • Histone-Modifying Enzymes:

    • Induce covalent modifications affecting histone core binding with DNA.

Factors in Modifying Chromatin and Histones

  • Histone acetylation serves to decrease positive charges, thus loosening DNA-histone connections.

  • Other modifications include lysine and arginine methylation and serine phosphorylation.

  • Epigenetics: Studies gene expression regulation through various modifications without changing DNA sequences.

Response Elements in Eukaryotes

  • Specific enhancers respond to metabolic factors like heat-shock or glucocorticoid elements, crucial for inducing gene transcription under varying cellular conditions.

Structural Motifs in DNA-Binding Proteins

  • Transcription factors typically have two functional domains:

  1. DNA-binding Domain: Interacts directly with DNA, can include motifs such as:

    • Helix-turn-helix (HTH) structure.

    • Zinc finger.

    • Basic-region leucine zipper (bZIP).

  2. Transcription-Activation Domain: Engages with protein complexes instead of directly with DNA.

Helix–Turn–Helix (HTH)

  • The HTH motif has an α-helix segment fitting into the major groove of DNA, typically consisting of conserved amino acids.

Zinc Fingers

  • Composed of nine repetitive 30 amino acid structures, stabilized by zinc ions binding to conformational folds, allowing for major groove interactions.

Basic-Region Leucine Zipper Motif

  • Frequently found in transcription factors, this motif consists of basic amino acids interacting with the DNA backbone, with leucines forming hydrophobic pockets to facilitate protein dimerization.

Posttranscriptional RNA Modifications

  • Processing of tRNA, rRNA, and mRNA occurs post-transcription, transforming them into functional forms.

  • Type of Modifications:

    • Trimming leader and trailer sequences.

    • Adding terminal sequences post-transcription.

    • Modifying specific bases, especially in tRNA.

tRNA Modifications

  • Precursor tRNAs are transcribed in lengthy sequences, which are trimmed and modified to yield functional tRNAs, characterized by a CCA sequence at the 3' end for amino acid acceptance.

rRNA Modifications

  • Processing primarily involves methylation and trimming to achieve the right sizes.

mRNA Modifications

  • Features include:

    • 5′ Cap: Methylation of guanylate residue at the N-7 position.

    • Polyadenylation: Addition of 100-200 nucleotides to the 3′ end protecting mRNA from degradation by nucleases.

    • Exons and Introns: Exons are coding sequences, while introns are non-coding sequences that get spliced out during processing.

Splicing Reaction

  • Involves removing introns to form mature mRNA for translation.

  • This process requires specific splice site sequences and the involvement of small nuclear ribonucleoproteins (snRNPs), forming a lariat from the intron.

Alternative RNA Splicing

  • Controls gene expression at the splicing level, creating protein isoforms through different mRNA forms depending on cellular requirements and regulatory proteins affecting splice site recognition.

Noncoding RNAs (NcRNAs)

  • Comprise 98% of transcriptional output in human genomes and lack protein-coding abilities, playing roles in various biological processes.

    • Micro RNA (miRNA): Involved in gene expression regulation and developed internally.

    • Small interfering RNA (siRNA): Controls gene expression via suppression, often originating from external sources, including viruses.

Ribozymes

  • Catalytic RNA capable of self-splicing and involved in protein synthesis.

  • Ribozymes are less efficient than protein catalysts, but laboratory ribozymes can catalyze multiple reactions and regenerate, whereas in vivo they predominantly act once.

Types of Ribozymes

  • Include structures like the hammerhead and hairpin ribozymes, functioning in RNA processing and catalysis.

Review of Key Terms and Concepts

  • DNA: Double-stranded molecule serving as genetic material.

  • Enhancer: DNA sequence boosting transcription levels.

  • Silencer: Sequence that inhibits transcription levels.

  • Promoter: Region where transcription begins.

  • Transcription: Process of synthesizing RNA from a DNA template.

  • Pre-mRNA: Initial RNA transcript modified into mature mRNA post-transcription through processes like splicing and polyadenylation.

  • 3' UTR: Untranslated region at the end of mRNA transcript affecting stability and translation efficiency.