Overview
This lecture introduces the basics of transcriptional regulation in prokaryotes and eukaryotes, focusing on transcription factors, sigma factors, consensus sequences, and the operon concept as preparation for studying the trp operon.
Introduction to Transcriptional Regulation
- Transcriptional regulation controls gene expression by influencing how and when genes are transcribed.
- RNA polymerase cannot bind DNA on its own and requires transcription factors for promoter recognition.
- Both prokaryotes and eukaryotes use transcription factors, though the mechanisms and factors differ slightly.
Types of DNA-Binding Proteins
- Non-sequence specific DNA-binding proteins, such as histones and RNA polymerase, bind DNA without sequence specificity.
- Sequence-specific DNA-binding proteins, like most transcription factors, recognize and bind particular DNA sequences.
Transcription Factors and Their Roles
- General transcription factors are required for RNA polymerase binding but are not promoter-specific.
- Regulatory transcription factors direct RNA polymerase to specific genes at specific times and can act as positive (activating) or negative (repressing) regulators.
- In prokaryotes, sigma factors serve as general transcription factors, while in eukaryotes, the basal transcription factor complex fulfills this role.
Sigma Factors in Prokaryotes
- Sigma factors in E. coli (e.g., sigma 70) bind RNA polymerase, forming the holoenzyme, allowing promoter recognition.
- Sigma 70 is the housekeeping sigma, regulating general gene expression; other sigma factors respond to specific stress conditions.
- Sigma contacts DNA at the -10 (Pribnow box) and -35 regions of the promoter, which are recognized as consensus sequences.
Consensus Sequences
- Consensus sequences are derived from frequency analysis of nucleotides at binding sites across various promoters.
- These sequences represent the most common nucleotide at each position but individual promoter sequences may vary.
- Sigma 70 binds different promoters with varying affinities based on sequence similarity to the consensus.
Operons
- An operon is a cluster of genes transcribed together from a single promoter, common in prokaryotes.
- The resulting mRNA contains multiple protein-coding regions (structural genes), each with its own ribosome binding site, start, and stop codon.
- Operons allow coordinated regulation of genes involved in a shared metabolic pathway.
Key Terms & Definitions
- Transcriptional Regulation — control of gene expression at the transcription stage.
- Transcription Factor — protein that helps RNA polymerase bind to promoters; can be general or regulatory.
- Sigma Factor — a general transcription factor in prokaryotes needed for promoter recognition.
- Consensus Sequence — the most frequently observed nucleotide sequence at each position of multiple binding sites.
- Operon — a group of genes transcribed together from one promoter, resulting in a single mRNA.
Action Items / Next Steps
- Review the definition and function of operons.
- Watch recommended animations on the trp operon for visualization.
- Prepare for the next video on the specific regulation mechanisms of the trp operon.
Overview
This lecture explains how E. coli regulates transcription of the tryptophan (trp) operon through two mechanisms: repression by the TrpR repressor and attenuation based on tryptophan-tRNA levels.
Structure and Function of the Trp Operon
- The trp operon consists of five structural genes (trpE, trpD, trpC, trpB, trpA) encoding enzymes needed for tryptophan biosynthesis.
- The operon includes a promoter, operator, leader sequence (for attenuation), and structural genes.
- E. coli uses the trp operon to synthesize tryptophan when external supplies are low.
Transcriptional Regulation by TrpR Repressor
- The trpR gene encodes the TrpR repressor protein, produced upstream of the operon.
- TrpR binds to the operator region and blocks RNA polymerase when activated.
- TrpR requires tryptophan (co-repressor) to bind DNA; when tryptophan is abundant, the operon is repressed.
- When tryptophan levels drop, less TrpR is bound to DNA, allowing transcription.
Mechanism of Attenuation (Early Termination)
- Attenuation is a second regulatory mechanism controlled by tryptophan-tRNA levels.
- The leader sequence of the mRNA contains a short peptide region with two adjacent tryptophan codons.
- Three possible hairpin (stem-loop) structures can form: 1-2, 2-3, and 3-4.
- When tryptophan-tRNA is abundant, translation proceeds, the ribosome covers region 2, and regions 3-4 form a hairpin, causing early transcription termination.
- When tryptophan-tRNA is scarce, the ribosome stalls at the tryptophan codons (region 1), allowing a 2-3 hairpin to form, preventing the 3-4 terminator hairpin and permitting full transcription.
RNA Structure and Hairpins
- RNA can form intramolecular base-pairing (hairpin/stem-loop structures) due to single-stranded regions.
- Hairpin loops are crucial in the attenuation mechanism at the trp operon and in other RNA processes.
Summary of Regulatory Logic
- The operon is only transcribed when there is a lack of tryptophan or tryptophan-tRNA, ensuring resource efficiency.
- Two independent but coordinated regulatory layers (repression and attenuation) finely tune gene expression in response to nutrient availability.
Key Terms & Definitions
- Operon — Cluster of genes under control of a single promoter in prokaryotes.
- Repressor — Protein that inhibits gene transcription by binding DNA at the operator.
- TrpR — The repressor protein that regulates the trp operon.
- Attenuation — Regulatory mechanism causing early termination of transcription.
- Hairpin (stem-loop) — Secondary structure formed by base pairing within single-stranded RNA.
- Co-repressor — A molecule (e.g., tryptophan) that enables a repressor to bind DNA.
Action Items / Next Steps
- Review the lecture slides and recommended animations for the trp operon attenuation.
- Prepare for the quiz by understanding both repression and attenuation mechanisms.
- Look up the structural differences between RNA and DNA if unclear.
- Next lecture: non-coding RNAs and RNA interference (RNAi).
Overview
This lecture introduces non-coding RNAs, highlighting their roles and types, and sets the stage for detailed discussion on RNA interference and gene regulation via microRNAs (miRNAs) and small interfering RNAs (siRNAs).
Non-Coding RNA: General Concepts
- Non-coding RNAs (ncRNAs) are RNAs that do not code for proteins.
- RNAs can base pair with themselves or with other nucleic acids, forming complex 3D structures.
- The extra oxygen on RNA’s ribose sugar allows it to participate in catalytic reactions, making some RNAs catalytic (ribozymes).
Types and Functions of Non-Coding RNAs
- Ribosomal RNA (rRNA) is a catalytic component of ribosomes and catalyzes peptide bond formation during protein synthesis.
- Transfer RNA (tRNA) carries amino acids to the ribosome and ensures correct amino acid sequence via base pairing with mRNA.
- MicroRNA (miRNA) and small interfering RNA (siRNA) regulate gene expression post-transcriptionally, primarily by silencing specific mRNAs.
- Small nuclear RNA (snRNA) is part of the spliceosome, catalyzing RNA splicing to process pre-mRNA in eukaryotes.
- Piwi-interacting RNAs (piRNAs) regulate gene expression during sperm formation.
- Long non-coding RNAs (lncRNAs), such as XIST and TSIX, regulate processes like X chromosome inactivation.
- CRISPR RNA (crRNA) is part of a bacterial defense system, guiding proteins to target and destroy viral DNA.
- Signal recognition particle (SRP) RNA helps direct proteins to the endoplasmic reticulum during synthesis.
- RNA nucleases are ribonucleoproteins where the RNA component acts as a ribozyme to process or degrade other RNA molecules.
RNA Interference (RNAi) Overview
- miRNAs and siRNAs act as post-transcriptional regulators, affecting mRNA stability, translation, and localization.
- RNAi was discovered by Andrew Fire and Craig Mello, who won the Nobel Prize in 2006.
- In humans, siRNAs often originate from exogenous (external) sources such as viruses, while miRNAs are encoded in the genome.
- Over 60% of human protein-coding genes are regulated by miRNAs.
Key Terms & Definitions
- Non-coding RNA (ncRNA) — RNA molecules not translated into proteins.
- Ribozyme — An RNA molecule with catalytic activity.
- MicroRNA (miRNA) — Small endogenous RNA that regulates gene expression post-transcriptionally.
- Small interfering RNA (siRNA) — Small RNA, often from external sources, that silences target mRNAs.
- Spliceosome — A complex for removing introns from pre-mRNA, containing snRNAs.
- Long non-coding RNA (lncRNA) — Longer RNA molecules with regulatory functions, often in gene expression.
- CRISPR RNA (crRNA) — RNA guiding bacterial proteins to destroy foreign DNA.
Action Items / Next Steps
- Read the Nobel Prize "Popular Information" and "Advanced Information" summaries about RNA interference.
- Prepare for the next lecture on the detailed mechanisms of RNAi, focusing on miRNA and siRNA pathways.
Overview
This lecture covers RNA interference (RNAi) as a mechanism of gene expression regulation in eukaryotes, focusing on the roles of microRNAs (miRNAs) and small interfering RNAs (siRNAs).
RNA Interference Overview
- RNA interference (RNAi) is a gene regulation mechanism affecting mRNA stability and translation in eukaryotes.
- RNAi uses small RNA molecules to silence or repress gene expression.
MicroRNA (miRNA) Pathway
- miRNAs are encoded in the genome and transcribed by RNA polymerase as primary miRNAs (pri-miRNAs).
- pri-miRNAs are processed into precursor miRNAs (pre-miRNAs) by Drosha in the nucleus.
- Pre-miRNAs are exported to the cytosol and further processed by Dicer into short double-stranded RNA duplexes.
- The RNA-Induced Silencing Complex (RISC) incorporates one RNA strand (the guide strand) and degrades the other.
- The guide strand base pairs with target mRNA to direct gene silencing.
- Perfect base pairing activates Argonaute (a RISC component) for mRNA cleavage and degradation.
- Incomplete base pairing typically results in translational repression or mRNA sequestration in the P-body for storage or degradation.
- miRNAs can target multiple mRNAs, especially with imperfect complementarity.
Small Interfering RNA (siRNA) Pathway
- siRNAs usually originate from exogenous sources, like viral RNA entering the cell.
- Dicer processes viral double-stranded RNA into siRNA duplexes in the cytosol.
- RISC uses one strand as a guide to target viral mRNAs with perfect base pairing, leading to Argonaute-mediated cleavage.
- siRNA provides an antiviral defense by degrading viral mRNAs.
Research and Therapeutic Applications
- RNAi is a powerful research tool for "knockdown" of gene expression by introducing synthetic siRNAs complementary to target mRNAs.
- Used to study gene function by observing effects of reduced protein expression.
- RNAi-based therapies are emerging but face delivery and immune response challenges.
- A few FDA-approved RNAi therapies now exist.
Comparison: miRNA vs. siRNA Pathways
- miRNAs are endogenously transcribed, processed first by Drosha, then Dicer, and may imperfectly bind targets.
- siRNAs are exogenous, processed only by Dicer, and generally perfectly match their targets, leading to direct cleavage.
Key Terms & Definitions
- RNA interference (RNAi) — Regulation of gene expression using small RNAs to silence mRNA.
- MicroRNA (miRNA) — Endogenous small RNAs that regulate gene expression post-transcriptionally.
- Primary miRNA (pri-miRNA) — The initial transcript of miRNA, processed in the nucleus.
- Precursor miRNA (pre-miRNA) — Shorter form of miRNA after initial nuclear processing.
- Drosha — Nuclear nuclease that processes pri-miRNA to pre-miRNA.
- Dicer — Cytosolic nuclease that processes pre-miRNA and siRNA into RNA duplexes.
- RISC (RNA-Induced Silencing Complex) — Protein complex that mediates gene silencing using guide RNA.
- Argonaute — Catalytic component of RISC responsible for mRNA cleavage.
- Small interfering RNA (siRNA) — Short, double-stranded RNAs from exogenous sources that mediate RNAi.
- P-body — A cellular compartment where mRNAs can be stored or degraded.
Action Items / Next Steps
- Review the processing steps of both miRNA and siRNA pathways.
- Be prepared to discuss the challenges of RNAi-based therapies.
- Read ahead for the next topic in gene expression regulation.