Comprehensive Study Notes on Bacterial Transcription and RNA Polymerase Mechanics

Exam Logistics and Course Schedule

• Course schedule adjustments regarding transcription coverage:
  • Transcription is covered across two instructional days.
  • Bacterial transcription is covered first in its entirety, followed by eukaryotic transcription to enable direct comparison and contrast between the two systems.
• Scope of Exam 1:
  • Exam 1 covers Chapter $1$ through Chapter $5$.
  • Chapter $1$ serves as the introductory chapter.
  • Chapter $5$ will be completed prior to the exam.
  • Chapter $6$ relates to translation and naturally integrates into translation topics; it will be introduced briefly following Chapter $5$.
  • No new instructional material will be presented on the morning immediately preceding the exam.

Overview of Bacterial Transcription

• Transcription is standardly abbreviated as TX.
• Definition of Promoter:
  • A promoter is a specific DNA sequence that signals the starting location for transcription.
• Mechanism in Bacteria vs. Eukaryotes:
  • Bacterial transcription mechanisms are relatively straightforward, highly defined, and well-characterized, primarily based on models studied in Escherichia coli (E. coli).
  • Variations on these mechanisms occur across different bacterial species.
• Stages of Transcription:
  • Initiation (the beginning phase).
  • Elongation (the middle phase).
  • Termination (the ending phase).

RNA Polymerase Holoenzyme Structure and Gel Electrophoresis

• Holoenzyme Composition:
  • The complete bacterial RNA polymerase holoenzyme consists of $5$ polypeptide subunits.
  • Structural subunits include:
  • Two identical alpha ($\alpha$) subunits.
  • Two large subunits close in mass: beta ($\beta$) and beta prime ($\beta'$).
  • One sigma ($\sigma$) subunit, which specifically functions to bind and bring in the DNA template by recognizing the promoter sequence.
  • The core enzyme consists of the catalytic subunits without the $\sigma$ subunit; addition of the $\sigma$ subunit forms the active holoenzyme.
• X-ray Crystallography of RNA Polymerase:
  • Structural determination relies on X-ray crystallography, a technique developed over the past $70$ years.
  • Structural models (including false-color crystal structures approximately $20$ years old) depict the core enzyme complexed with a crystallized fragment of DNA, showing the $\sigma$ subunit directly grasping the DNA.
• Electrophoretic Separation of Subunits:
  • Polyacrylamide protein gel electrophoresis separates proteins based on molecular length/size under an electrical field.
  • Smaller polypeptides migrate faster through the gel matrix than larger polypeptides:
  • The smaller $\alpha$ subunits migrate faster than the larger $\beta$ subunits.
  • Standard gel electrophoresis of the holoenzyme yields $3$ distinct resolved bands.
  • Resolution of $\beta$ and $\beta'$ Subunits:
  • Running an extended gel over a longer duration successfully separates the similar-sized $\beta$ and $\beta'$ subunits into distinct bands.
  • Alternatively, protein bands can be excised from the gel and analyzed via mass spectrometry, which cleaves peptides into fragments to determine molecular identity.
  • Running extended gels is functionally simpler and significantly more cost-effective than using specialized mass spectrometers (such as specialized instruments beyond standard laboratory equipment like Dr. Chandler's mass spectrometer).
• Spatial Orientation:
  • Transcription is conventionally drawn from left to right, indicated by a bent arrow at the promoter sequence.
  • Within a living cell in three-dimensional space, gravitational forces are irrelevant at the molecular scale; thus, the spatial orientation of the $\sigma$ subunit (whether depicted on top or bottom) does not alter the mandatory $5'$ to $3'$ chemical directionality of synthesis.

Bacterial Promoter Structure and Consensus Sequences

• Consensus Sequence Concept:
  • A consensus sequence represents the calculated average or most common nucleotide sequence derived from aligning corresponding region sequences across multiple genes.
  • Individual promoters may match the consensus sequence exactly or display slight sequence variations.
  • E. coli possesses approximately $2{,}000$ genes, which represents roughly $10\%$ of the estimated $20{,}000$ to $25{,}000$ genes present in the human genome.
• Key E. coli Promoter Elements:
  • The $-10$ sequence ($-10$ box):
  • Consensus sequence is TATAAT.
  • Composed entirely of adenine ($\text{A}$) and thymine ($\text{T}$) bases.
  • Serves as the precise site where DNA strand unwinding is initiated, as $\text{A-T}$ base pairs possess fewer hydrogen bonds than $\text{G-C}$ pairs and unwind more readily.
  • The $-35$ sequence ($-35$ box):
  • Consensus sequence is TTGACA.
  • The inverted or reverse sequence TGTCAA is non-equivalent and does not function identically.
• Promoter Numbering Convention:
  • The transcriptional start site—the exact nucleotide base where transcription begins—is designated as $+1$.
  • There is no base $0$ in the sequence numbering scheme.
  • The nucleotide base immediately preceding $+1$ is designated as $-1$.
  • The $-10$ box is located on average $10$ bases upstream of the $+1$ start site.
  • The $-35$ box is located on average $35$ bases upstream of the $+1$ start site.
  • An average E. coli promoter spans approximately $35$ base pairs in length.

Mechanism of Transcription Initiation and Strand Specificity

• Template Strand vs. Coding Strand:
  • Template Strand: The physical DNA strand that is directly read and copied by RNA polymerase.
  • Coding Strand: The non-template DNA strand whose base sequence matches the resulting RNA transcript (with uracil replacing thymine).
  • RNA complementary base pairing rules apply during synthesis (e.g., a template thymine ($\text{T}$) directs the insertion of an adenine ($\text{A}$) into the RNA transcript).
  • Example: A template DNA sequence of 5'-CGTA-3' dictates an RNA sequence of 3'-GCAU-5' (or 5'-UACG-3').
• Identifying Strand Identity in Genomes:
  • Neither DNA strand is exclusively the coding or template strand genome-wide; individual genes on the same chromosome can utilize opposite strands as their template.
  • Determining which strand serves as the template requires analyzing the physical gene product (mRNA transcript or encoded protein sequence, such as locating the start codon ATG).
  • Genomic annotation utilizes automated algorithms (such as shotgun sequencing compilation where short DNA reads are assembled) to identify candidate $-35$, $-10$, transcriptional start, and translational start sites (e.g., analyzing newly isolated bacterial strains from environmental sewage samples, such as from Gua). Experimental biochemistry is required to confirm active expression of a gene product.
• Independence from Primers:
  • Unlike DNA polymerases (which require a pre-existing primer providing a free $3'$ hydroxyl end), RNA polymerases do not require a primer to initiate de novo polynucleotide synthesis.
• Summary Steps of Initiation:
  1. RNA polymerase holoenzyme binds to the promoter sequence at the $-35$ and $-10$ regions via the $\sigma$ subunit.
  2. The enzyme unwinds the double-stranded DNA at the $-10$ $\text{A-T}$ rich region, transitioning from a closed to an open promoter complex.
  3. RNA polymerase initiates RNA synthesis at the $+1$ nucleotide position.

RNA Synthesis Chemistry and Elongation

• Polymerization Reaction Mechanics:
  • RNA synthesis proceeds exclusively in the $5'$ to $3'$ direction.
  • Incoming substrate nucleotides are ribonucleotide triphosphates containing ribose sugars.
  • The $3'$ hydroxyl ($\text{OH}$) group of the growing RNA chain attacks the alpha-phosphate of the incoming ribonucleotide triphosphate.
  • The reaction forms a phosphodiester bond and liberates a molecule of pyrophosphate ($\text{P}_\text{i}\text{-P}_\text{i}$, composed of two covalently linked phosphate groups).
  • Pyrophosphate cleavage releases substantial free energy (denoted by the prefix pyro-, meaning fire) and is recycled by cellular pathways.
• Elongation Process:
  • RNA polymerase moves along the template DNA, continuously unwinding the duplex ahead of the active site and adding complementary ribonucleotides to the $3'$ end of the growing RNA transcript.

Bacterial Transcription Termination

• Hairpin-Dependent (Rho-Independent / Intrinsic) Termination:
  • As RNA polymerase transcribes the terminal region of a gene, it synthesizes a sequence rich in guanine ($\text{G}$) and cytosine ($\text{C}$) bases, followed by a non-complementary loop region and a reverse complementary $\text{G-C}$ rich segment.
  • The complementary $\text{G-C}$ rich regions on the single-stranded RNA transcript self-hybridize, snapping together to form a stable stem-loop (hairpin) secondary structure.
  • The strong hydrogen bonding within the $\text{G-C}$ hairpin structure physically stalls and destabilizes the RNA polymerase complex, causing the enzyme to dissociate from the DNA template and release the completed RNA transcript.

Questions and Student Discussion

• Question: Is the subunit diagram inverted or upside down between different representations?
  • Answer: Spatial orientation in diagrams is arbitrary. Because gravity is not a functional force at the molecular level within a cell, 3D spatial alignment does not affect the process. Transcription proceeds along a specific chemical direction ($5'$ to $3'$) regardless of visual representation.
• Question: Why are promoter sequences designated with negative numbers like $-10$ and $-35$?
  • Answer: Negative numbers represent counting upstream (backwards) from the transcriptional start site, which is designated as $+1$. Because there is no base $0$, the base directly preceding $+1$ is $-1$. Thus, $-10$ and $-35$ represent sites located on average $10$ and $35$ bases prior to the start site.
• Question: How do you determine which strand is the coding strand versus the template strand?
  • Answer: Strand determination requires examining the actual gene product (mRNA or protein sequence). Within a single gene, only one strand acts as the template, but across an entire genome, either strand can serve as the template for different genes. On introductory examinations, strand designations ($5'$ to $3'$ directionality and template/coding identity) are explicitly specified.
• Question: Is the promoter region in E. coli typically around $35$ bases long?
  • Answer: Yes, an average E. coli promoter spans approximately $35$ bases. Eukaryotic promoters differ significantly in size and complexity.
• Question: How are the similar-sized $\beta$ and $\beta'$ subunits resolved if standard gels only show $3$ bands?
  • Answer: Standard short gel runs group $\beta$ and $\beta'$ into a single band due to their close molecular weight. Running an extended gel over a longer time separates them into distinct bands. Mass spectrometry can also identify them by peptide fragmentation, though extended gel runs are simpler and cheaper.