RNA Transcription and Processing

Structural and Chemical Properties of RNA

• Comparison Between RNA and DNA Architecture:

  • RNA typically exists as a single-stranded polyribonucleotide chain, contrasting with the double-stranded helical structure of DNA.

  • The single-stranded nature confers high conformational flexibility, enabling RNA molecules to fold into intricate three-dimensional structures through intramolecular complementary base pairing.

  • Intramolecular folding and tertiary structure dictate the distinct functional capabilities of various RNA species.

• Pentose Sugar Chemistry:

  • RNA incorporates ribose as its five-carbon pentose sugar, whereas DNA contains 2'-deoxyribose.

  • Ribose possesses a reactive hydroxyl group ($-OH$) attached to the $2'$ carbon position ($2'\text{-OH}$), whereas deoxyribose possesses only a hydrogen atom ($-H$) at this position.

• Nitrogenous Bases and Hydrogen-Bonding Properties:

  • RNA comprises four primary bases: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Uracil ($U$). Uracil replaces the thymine ($T$) found in DNA.

  • Uracil is a pyrimidine that pairs with Adenine through two hydrogen bonds ($A=U$).

  • During tertiary RNA folding, Uracil can also form non-Watson-Crick base pairs with Guanine ($G-U$ pairing). These $G-U$ wobble pairs are energetically weaker than standard $A-U$ base pairs and occur exclusively during structural RNA folding, never during transcription.

• Ribonucleotide Monomers:

  • Adenosine $5'$-monophosphate ($\text{AMP}$): contains adenine bound to ribose and a single $5'$ phosphate.

  • Guanosine $5'$-monophosphate ($\text{GMP}$): contains guanine bound to ribose and a single $5'$ phosphate.

  • Cytidine $5'$-monophosphate ($\text{CMP}$): contains cytosine bound to ribose and a single $5'$ phosphate.

  • Uridine $5'$-monophosphate ($\text{UMP}$): contains uracil bound to ribose and a single $5'$ phosphate.

• Catalytic Activity of RNA (Ribozymes):

  • Unlike double-stranded DNA, RNA molecules can fold into catalytic configurations capable of enzymatically accelerating specific biochemical reactions.

  • Catalytic RNA molecules are termed ribozymes and function analogously to protein enzymes.

Functional Classes and Abundance of Cellular RNA

• Primary Functional Divisions:

  • Messenger RNA (mRNA): Transcribes and carries the genetic coding sequence from chromosomal DNA to the ribosome for polypeptide synthesis. Functions as an intermediate in protein-coding gene expression.

  • Functional (Non-Coding) RNAs: RNA molecules that do not code for proteins; the folded RNA transcript itself is the ultimate functional product.

• Classes of Functional RNA Molecules:

  • Transfer RNA (tRNA): Adapter molecules present in both bacterial and eukaryotic cytoplasm; delivers specific amino acids to the ribosome corresponding to codons along the mRNA template during translation.

  • Ribosomal RNA (rRNA): Structural and catalytic constituents of the macromolecular ribosome complex in both prokaryotes and eukaryotes; facilitates the alignment of mRNA and catalyzes peptide bond formation in the cytoplasm.

  • Small Nuclear RNA (snRNA): Confined to the eukaryotic nucleus; complexes with proteins to form small nuclear ribonucleoproteins (snRNPs) that drive pre-mRNA splicing within the spliceosome.

  • MicroRNA (miRNA): Eukaryotic regulatory RNAs acting within the cytoplasm; represses translation of specific target mRNAs or accelerates their degradation.

  • Small Interfering RNA (siRNA): Eukaryotic double-stranded regulatory RNAs operating in the cytoplasm; triggers targeted endonucleolytic cleavage and degradation of complementary RNA molecules.

  • Piwi-Interacting RNA (piRNA): Protects animal germline genomes by suppressing the mobilization of transposable genetic elements and viral sequences.

• Eukaryotic RNA Localization and Movement:

  • All eukaryotic RNA species are initially synthesized within the nucleus.

  • A 1957 pulse-chase experiment conducted by Elliot Volkin and Lazarus Astrachan utilizing radiolabeled uracil established that RNA is synthesized in the nucleus (pulse phase) and subsequently translocated into the cytoplasm (chase phase with nonradioactive precursors).

• Relative Abundance and Molecular Characteristics in Escherichia coli:

  • Ribosomal RNA (rRNA): Comprises approximately $80\%$ of total cellular RNA.

    • $23S\text{ rRNA}$: Sedimentation coefficient of $23S$, molecular mass of $1.2 \times 10^3\,kd$, length of $3700\text{ nucleotides}$.

    • $16S\text{ rRNA}$: Sedimentation coefficient of $16S$, molecular mass of $0.55 \times 10^3\,kd$, length of $1700\text{ nucleotides}$.

    • $5S\text{ rRNA}$: Sedimentation coefficient of $5S$, molecular mass of $3.6 \times 10^1\,kd$, length of $120\text{ nucleotides}$.

  • Transfer RNA (tRNA): Comprises approximately $15\%$ of total cellular RNA; sedimentation coefficient of $4S$, molecular mass of $2.5 \times 10^1\,kd$, length of $75\text{ nucleotides}$.

  • Messenger RNA (mRNA): Comprises approximately $5\%$ of total cellular RNA; exhibits heterogeneous sedimentation coefficients, molecular masses, and nucleotide lengths.

Molecular Mechanisms of Prokaryotic Transcription

• Principles of Template Reading and Polymerization:

  • In Escherichia coli, a single RNA polymerase synthesizes all cellular RNA species.

  • Transcription relies on complementary base pairing against a locally unzipped DNA double helix.

  • Transcription is asymmetrical: while either DNA strand can theoretically serve as a template, only one designated strand (the template strand) is transcribed for any given gene.

  • The non-transcribed DNA strand is termed the nontemplate or coding strand because its sequence corresponds directly to the resulting RNA transcript (with thymine substituted by uracil).

  • RNA polymerase reads the template strand in the $3' \rightarrow 5'$ direction and synthesizes the nascent RNA chain antiparallel in the $5' \rightarrow 3'$ direction by adding successive ribonucleotides to the free $3'\text{-OH}$ terminus.

  • Multiple RNA polymerases can transcribe a single gene simultaneously in tandem, generating characteristic Christmas-tree-like transcription patterns visible by electron microscopy.

• Bacterial Promoter Architecture and Consensus Sequences:

  • Transcription initiation requires the recognition of conserved promoter sequences situated upstream of the transcription start site ($+1$).

  • The sequence preceding $+1$ is designated upstream (negative integers), whereas sequence after $+1$ is designated downstream (positive integers).

  • The region between the $+1$ initiation site and the translation initiation codon ($\text{ATG}$) forms the $5'$ untranslated region ($5'\text{ UTR}$).

  • Two critical conserved promoter elements in E. coli include:

    • $-35\text{ Region}$: Consensus sequence 5'-TTGACA-3'.

    • $-10\text{ Region (Pribnow Box)}$: Consensus sequence 5'-TATAAT-3', situated roughly $10\text{ base pairs}$ upstream of the transcription initiation site.

    • The intervening spacing separating the $-35$ and $-10$ consensus elements is strictly maintained at $15\text{--}17\text{ base pairs}$.

• Subunit Organization of Bacterial RNA Polymerase Holoenzyme:

  • The complete functional enzyme complex (holoenzyme) comprises the five-subunit core enzyme plus an interchangeable regulatory sigma ($\sigma$) factor:

    • $\alpha\text{ Subunits (two copies)}$: Coordinate overall enzyme assembly and mediate physical interactions with regulatory proteins.

    • $\beta\text{ Subunit}$: Catalyzes phosphodiester bond formation.

    • $\beta'\text{ Subunit}$: Confers non-specific affinity for binding to the DNA template.

    • $\omega\text{ (omega) Subunit}$: Maintains structural integrity, aids enzyme assembly, and participates in gene expression regulation.

    • $\sigma\text{ (sigma) Factor (e.g., } \sigma^{70}\text{)}$: Recognizes and specifically binds the $-35$ and $-10$ promoter elements to position the holoenzyme; melts the DNA double helix across the $-10$ region to form an open complex, and dissociates from the core enzyme following initiation.

• Elongation and Enzymatic Mechanism:

  • As RNA polymerase translocates along the template, it unwinds the double helix ahead of the catalytic center while rewinding the transcribed DNA behind it, establishing a transient, moving single-stranded transcription bubble.

  • Within the bubble, incoming ribonucleoside triphosphates ($\text{NTPs}$) base-pair with the template strand.

  • The $3'\text{-OH}$ group of the nascent RNA strand carries out a nucleophilic attack on the inner $\alpha$-phosphate of the incoming $\text{NTP}$, forming a new $3' \rightarrow 5'$ phosphodiester linkage and releasing inorganic pyrophosphate ($\text{PP}_i$):

$\text{NTP} + (\text{NMP})_n \rightarrow (\text{NMP})_{n+1} + \text{PP}_i$

• Prokaryotic Termination Mechanisms:

  • Transcription extends past the protein-coding sequence into the $3'$ untranslated region ($3'\text{ UTR}$) until termination sequences are encountered.

  • Intrinsic (Rho-Independent) Termination:

    • The RNA sequence encodes an inverted repeat rich in guanine and cytosine ($G-C$) followed by a run of approximately eight uracil residues (CUUUUUUUU 3').

    • The transcribed inverted repeat folds into a stable intramolecular GC-rich hairpin (stem-loop) structure.

    • The mechanical stress from hairpin formation stalls the polymerase over the weak, unstable adenine-uracil ($A-U$) hybrid tract, leading to transcript release.

• Rho-Dependent Termination:

  • Does not rely on a poly-U tract or stable hairpin loops.

  • A hexameric protein known as the Rho factor binds to an unstructured, cytosine-rich sequence on the nascent RNA termed the Rho utilization site (rut site).

  • Rho translocates along the RNA strand following the polymerase.

  • When RNA polymerase pauses at a specific termination sequence, Rho catches up, unwinds the DNA-RNA hybrid within the transcription bubble via intrinsic helicase activity, and releases the polymerase, template, and nascent RNA.

Eukaryotic Transcription and General Transcription Factors

• Compartmentalization Differences Between Prokaryotes and Eukaryotes:

  • Prokaryotes lack a nuclear envelope; transcription and translation occur simultaneously within the same cytoplasmic compartment (coupled transcription-translation).

  • Eukaryotes feature strict spatial and temporal separation: transcription and pre-mRNA processing occur entirely inside the nucleus, whereas translation occurs in the cytoplasm following mRNA export.

• Distinct Eukaryotic RNA Polymerases:

  • RNA Polymerase I: Localized to the nucleolus; synthesizes the primary precursor encoding the large ribosomal RNAs ($28S$, $18S$, and $5.8S\text{ rRNA}$), representing all rRNAs except $5S\text{ rRNA}$.

  • RNA Polymerase II: Localized to the nucleoplasm; transcribes all nuclear protein-encoding genes into messenger RNA precursors (pre-mRNA), as well as specific small non-coding RNAs (snRNAs and miRNAs). Operates as a massive multimeric complex exceeding $10\text{ subunits}$.

  • RNA Polymerase III: Localized to the nucleoplasm; transcribes all transfer RNAs (tRNAs), the $5S\text{ rRNA}$, and select small nuclear RNAs.

  • Mitochondrial RNA Polymerase (mt RNA Polymerase): Localized within mitochondria; transcribes mitochondrial genes into mtRNA.

• Conserved Eukaryotic Promoter Elements:

  • TATA Box (Goldberg-Hogness Box): Located approximately $-25\text{ to } -30\text{ base pairs}$ upstream of the transcription start site; consensus sequence 5'-TATAAA-3'. Essential for local unwinding and positioning of the initiation complex.

  • CAAT Box: Situated near $-75\text{ to } -80\text{ base pairs}$; consensus sequence 5'-GGNCAATCT-3' or 5'-GGCCAATCT-3'.

  • GC Box: Located around $-90\text{ base pairs}$ (also found at positions $-60$ and $-100$); consensus sequence 5'-GGGCGG-3'.

  • Octamer Box: Located between $-120\text{ and } -140\text{ base pairs}$; consensus sequence 5'-ATTTGCAT-3'.

• Preinitiation Complex Assembly and Initiation:

  • Eukaryotic RNA polymerases cannot recognize promoters or initiate transcription independently; they require assembly of General Transcription Factors (GTFs).

  • Transcription factor IID ($\text{TFIID}$), containing the TATA-binding protein ($\text{TBP}$), binds the TATA box sequence located $30\text{ base pairs}$ upstream of the start site.

  • Binding of $\text{TBP}$ induces recruitment of additional transcription factors ($\text{TFIIB}$, $\text{TFIIF}$, $\text{TFIIE}$, and $\text{TFIIH}$) alongside the core RNA Polymerase II enzyme to construct the preinitiation complex ($\text{PIC}$).

  • $\text{TFIIH}$ mediates ATP-dependent unwinding of promoter DNA and phosphorylates the carboxy-terminal domain ($\text{CTD}$) of RNA Polymerase II, triggering promoter clearance and productive transcriptional elongation.

Co-transcriptional Pre-mRNA Processing and CTD Coordination

• Coordination by the Carboxy-Terminal Domain (CTD):

  • The carboxy-terminal domain ($\text{CTD}$) of the largest subunit ($\beta$-like subunit) of RNA Polymerase II acts as a central landing pad coordinating pre-mRNA processing events.

  • Reversible phosphorylation and dephosphorylation of amino acid residues within the heptapeptide repeats of the $\text{CTD}$ dynamically recruit specific enzyme complexes responsible for $5'$ capping, splicing, and $3'$ cleavage/polyadenylation.

  • RNA processing occurs co-transcriptionally while nascent pre-mRNA is actively emerging from the polymerase exit channel.

• The Three Core Processing Modifications:

  • $5'$ 7-Methylguanosine Capping: Capping enzymes recruited to the phosphorylated $\text{CTD}$ add a 7-methylguanosine cap to the $5'$ end of the nascent pre-mRNA via an unusual $5' \rightarrow 5'$ triphosphate bridge shortly after transcription initiates. Protects mRNA from $5'$ exonucleases and serves as a critical ribosome binding signal during translation initiation.

  • Intron Removal (Splicing): Splicing machinery complexes assemble directly onto the emerging transcript via recruitment by phosphorylated residues on the $\text{CTD}$, precisely excising non-coding introns and ligating exons.

  • $3'$ Cleavage and Polyadenylation: Polyadenylation factors associate with the $\text{CTD}$ to recognize polyadenylation signals, endonucleolytically cleave the transcript, and add a terminal poly-A tail consisting of $150\text{ to } 200\text{ adenine residues}$. Confers transcript stability and facilitates nuclear export.

Nuclear RNA Splicing, Spliceosome Assembly, and Alternative Splicing

• Gene Architecture (Exons vs. Introns):

  • Exons: Expressed regions containing coding sequences that dictate the primary amino acid sequence of proteins, along with terminal untranslated regions ($5'\text{ UTR}$ and $3'\text{ UTR}$).

  • Introns: Intervening, non-coding intragenic regions transcribed into pre-mRNA that must be excised prior to translation.

  • The 2000 Genome Research Conference highlighted that while single-celled yeast (Saccharomyces cerevisiae) possesses only $6300\text{ genes}$, human gene estimates ranged between $26{,}000\text{ and } 150{,}000\text{ genes}$ (later resolved to roughly $20{,}000\text{--}25{,}000$). The presence of introns and exons enables the generation of hundreds of thousands of distinct proteins ($100{,}000\text{s}$) from a modest gene inventory through alternative splicing.

• Catalytic Splicing Mechanisms and Self-Splicing Ribozymes:

  • Tom Cech (awarded the Nobel Prize in 1989) discovered that primary rRNA transcripts in Tetrahymena could excise an intervening $413\text{-nucleotide}$ intron autonomously in the complete absence of protein enzymes, establishing the concept of self-splicing ribozymes.

  • Nuclear pre-mRNA splicing within the spliceosome is catalytically executed by small nuclear RNA (snRNA) molecules rather than by protein components.

• Consensus Sequences of Splicing Junctions:

  • In genomic DNA, canonical splice junctions are designated GT...AG. In the transcribed pre-mRNA, canonical splice junctions are designated GU...AG.

  • $5'$ Splice Site: Lies at the junction between the upstream exon and the $5'$ end of the intron. Features the invariant consensus dinucleotide GU ($100\%$ conserved), flanked by the broader consensus 5'-(A/C)AG | GUA/GAGU-3'.

  • Branch Point: Located $15\text{ to } 45\text{ nucleotides}$ upstream of the $3'$ splice site; features a strictly conserved adenine ($A$, $100\%$ frequency) within the consensus 5'-CU(A/G)A(C/U)-3'.

  • $3'$ Splice Site: Marks the junction between the $3'$ end of the intron and the downstream exon. Features the invariant consensus dinucleotide AG ($100\%$ conserved), flanked by the consensus 5'-NCAGG-3'.

  • A genome-wide analysis of $1588$ predicted splice sites in Arabidopsis thaliana revealed that $1470\text{ sites}$ ($93\%$) adhered strictly to the canonical GT...AG rule.

• Spliceosome Structure and Two-Step Transesterification Chemistry:

  • The spliceosome is an RNA-protein machine composed of five small nuclear RNAs ($\text{U1}$, $\text{U2}$, $\text{U4}$, $\text{U5}$, and $\text{U6}$) combined with specialized proteins to form small nuclear ribonucleoprotein particles (snRNPs, pronounced "snurps").

  • Protein factors and kinases, including $\text{Prp31p}$, $\text{Prp1p}$, $\text{Prp4p}\text{ kinase}$, $\text{Cdc5p}$, and the $\text{Prp19}\text{ complex}$, mediate dynamic conformational rearrangements from tetra-snRNP to activated penta-snRNP assemblies.

  • First Transesterification Reaction: The nucleophilic $2'\text{-OH}$ group of the branch point adenosine attacks the phosphodiester bond at the $5'$ splice site (GU), cleaving the $5'$ exon-intron boundary and forming an unusual $2' \rightarrow 5'$ phosphodiester linkage that creates a looped lariat intermediate.

  • Second Transesterification Reaction: The newly liberated $3'\text{-OH}$ group of the upstream exon carries out a nucleophilic attack on the phosphodiester bond at the $3'$ splice site (AG), releasing the intron in the shape of a cowboy's lariat while ligating the adjacent exons together.

• Alternative Splicing and Proteomic Diversity:

  • Alternative splicing allows a single primary pre-mRNA transcript to generate multiple distinct mature mRNAs and polypeptide isoforms depending on cell type, developmental timing, or external stimuli.

  • Human genes express an average of 8 alternative splice variants.

  • Rat Tropomyosin Gene Paradigm: The pre-mRNA of the rat tropomyosin gene contains multiple exons (e.g., $1a$, $1b$, $2a$, $2b$, $3$, $4$, $5$, $6a$, $6b$, $7$, $8$, $9a$, $9b$, $9c$, $9d$) that are selectively included or excluded via alternative splicing across tissues:

    • Striated muscle: Excludes exons $1b$, $2a$, and $9d$, producing a muscle-specific isoform.

    • Smooth muscle: Selects distinct exon pairings including exon $2a$ and exon $9d$.

    • Brain: Expresses unique neurological isoforms such as $\text{TMBr-1}$, $\text{TMBr-2}$, and $\text{TMBr-3}$.

    • Fibroblasts: Utilizes alternate combinations yielding isoforms $\text{TM-2}$, $\text{TM-3}$, $\text{TM-5a}$, and $\text{TM-5b}$.

Small Non-Coding RNAs and RNA Interference (RNAi)

• Overview of RNA Interference (RNAi):

  • Small regulatory RNAs form a cellular pool of short double-stranded molecules measuring $21\text{ to } 24\text{ nucleotides (nt)}$ in length ($18\text{--}24\text{ base pairs}$ duplexes).

  • RNAi operates as a post-transcriptional gene silencing ($\text{PTGS}$) mechanism targeting mRNAs for translational inhibition or cleavage, as well as a transcriptional gene silencing ($\text{TGS}$) pathway inducing chromatin modifications.

  • Serves an essential defense role, protecting host genomes against parasitic nucleotide invaders such as retrotransposons and RNA/DNA viruses, while directing organismal developmental programming.

• MicroRNA (miRNA) Biogenesis and Mechanism:

  • miRNAs originate from endogenous non-protein-coding genes (e.g., the lin-4 gene in Caenorhabditis elegans).

  • Transcribed within the nucleus by RNA Polymerase II into primary transcripts that fold into stem-loop/hairpin structures.

  • Following preliminary nuclear processing, pre-miRNA is exported to the cytoplasm where Dicer endonucleolytically cleaves it into short double-stranded fragments (~$22\text{ nucleotides}$).

  • The duplex is loaded into the RNA-Induced Silencing Complex ($\text{RISC}$), which retains one strand as the biologically active guide strand while discarding the passenger strand.

  • miRNAs regulate different genes from those that produced them. The miRNA-loaded $\text{RISC}$ binds via partial or total complementarity to the $3'\text{ UTR}$ of target mRNAs (e.g., lin-14 or lin-28), promoting deadenylation, decay, or translational repression.

• Small Interfering RNA (siRNA) Biogenesis and Mechanism:

  • siRNAs originate from long, fully complementary double-stranded RNA ($\text{dsRNA}$) triggers derived from external sources (e.g., viral infection, transgenes) or bidirectional/antisense transcription.

  • Dicer cleaves long $\text{dsRNA}$ into short duplexes of $18\text{ to } 24\text{ base pairs}$.

  • Incorporated into $\text{RISC}$, which unwinds the duplex and presents the single-stranded guide strand.

  • Unlike miRNAs, siRNAs typically exhibit absolute sequence identity and target the same gene or foreign nucleic acid locus that produced them.

• Core Enzymatic Machinery of RNA Silencing:

  • Dicer (or Dicer-Like, DCL): A double-strand-specific ribonuclease (RNase III endonuclease) that measures and cleaves precursor $\text{dsRNA}$ into precise $21\text{--}24\text{ nucleotide}$ duplex products.

  • RNA-Induced Silencing Complex (RISC): A multi-protein effector complex that incorporates the single-stranded small RNA guide.

  • Argonaute (AGO) Protein: The core catalytic protein component of $\text{RISC}$. Contains an endonuclease active site ("slicer" activity) that cleaves the phosphodiester backbone of target mRNAs aligned opposite the guide strand, abolishing target protein expression.