rna

Ribonucleic Acids

Cellular RNA Synthesis

  • RNA synthesis from DNA is termed transcription.
  • The process is catalyzed by the large enzyme RNA polymerase, a fundamental enzyme present in all known life forms.

RNA Polymerase Requirements

Template

  • Substrate: Double-stranded DNA (dsDNA) serves as the substrate for RNA synthesis.
  • Transcription Dynamics:
    • Only one strand of the dsDNA is transcribed, known as the template strand.
    • The sequence of the template strand of DNA complements that of the RNA transcript.
    • The opposite strand, known as the coding strand, shares the same sequence as the RNA transcript (with thymine (T) in DNA replaced by uracil (U) in RNA).
  • Terminology:
    • Coding strand: Also referred to as the sense (+) strand.
    • Template strand: Also called the antisense (-) strand.

Activated Precursors

  • RNA synthesis requires building blocks in the form of ribonucleoside triphosphates (NTPs), specifically:
    • ATP (adenosine triphosphate)
    • GTP (guanosine triphosphate)
    • UTP (uridine triphosphate)
    • CTP (cytidine triphosphate)

Divalent Metal Ion

  • A divalent cation cofactor, notably Mg²⁺ or Mn²⁺, is essential for the proper functioning of RNA polymerase.
  • Reaction Mechanism:
    • The reaction catalyzed by RNA polymerase can be summarized as:
      (RNA)n + ext{ribonucleoside triphosphate} \leftrightarrow (RNA){n+1} + PPi

RNA Synthesis Dynamics

  • RNA synthesis parallels DNA synthesis in several aspects:
    • Directionality: Synthesis proceeds in the 5' to 3' direction.
    • Mechanism of Elongation: The mechanism mirrors that of DNA.
    • Driving Force: Synthesis is propelled by the hydrolysis of pyrophosphate (PPi).
  • Key Difference: Unlike DNA replication, RNA polymerase does not require a primer to initiate synthesis.

Genes as Transcriptional Units

  • A range of RNA types are produced by RNA polymerase from segments of DNA termed genes.
    • Genres differ in regulatory mechanisms:
    • Constitutive expression: Genes expressed continuously.
    • Regulated expression: Genes turned on or off under specific conditions.
  • The initiation of gene expression begins with transcription as RNA polymerase identifies gene start and stop sites on DNA.

Types of RNA Produced

  • There are three primary types of RNA synthesized:
    • Messenger RNA (mRNA): Encodes instructions for protein synthesis.
    • Transfer RNA (tRNA): Plays a critical role in the translation process of mRNA.
    • Ribosomal RNA (rRNA): Integral to ribosomal machinery required for translation.
  • In E. coli, all types of RNA are produced by the same RNA polymerase, while mammalian cells utilize distinct RNA polymerases but maintain similar chemical processes.

Stages of RNA Synthesis

  • RNA synthesis comprises three stages:
    1. Initiation
    2. Elongation
    3. Termination

Initiation of Transcription

  • Transcription begins at specific DNA sequences known as promoter sites.
  • Promoters guide RNA polymerase to the right initiation site for transcription.
    • Example Promoters:
    • -10 site (Pribnow box)
    • -35 sequence
    • Both of these are approximately 6 bp long and have consensus sequences that are recognized by RNA polymerase.

Consensus Sequences

  • Consensus sequences determine transcription start points.
  • Transcription initiation occurs at sites characterized by the specified sequences:
    • (-10 site example): TATAAT
    • Other examples for various sequences (A-D) typically contain GC-rich areas.

Efficiency of Promoters

  • Not all promoters perform equally:
    • Strong promoters: Transcription occurs frequently due to close matching of consensus sequences.
    • Weak promoters: Exhibit multiple nucleotide substitutions and result in lesser transcription frequency.
  • Efficiency of initial transcription is also regulated by transcription factors (proteins that bind near promoter sites and interact with RNA polymerase).

Role of Sigma Factors in RNA Polymerase

  • σ (sigma) subunit assists the RNA polymerase in locating the correct transcription start site by:
    • Significantly reducing the RNA polymerase's affinity for nonspecific DNA regions (by ~10,000-fold), facilitating rapid searching for promoters.
    • Preventing indiscriminate binding to DNA when the sigma subunit is absent, where RNA polymerase binds tightly due to a core enzyme's nature.
  • Once RNA chain synthesis begins, the nascent RNA interacts with the σ subunit, facilitating its ejection from the transcription complex.
  • This transition denotes the movement from initiation to elongation in transcription.

RNA Growth and Chain Elongation

  • RNA polymerase unwinds segments of DNA to reveal the template strand, unwinding approximately 17 bp at a time.
  • The process transitions from a closed promoter complex to an open complex enabling transcription.
  • Distinct tracts mark the 5' end of RNA strands with tags such as pppG or pppA.

Elongation Process

  • Following σ subunit loss, the core enzyme maintains firm affixation to the DNA template while continuing transcription until halting at a terminal signal.
  • The assembly, containing RNA polymerase, DNA, and the nascent RNA, is termed the transcription bubble, characterized by a locally denatured region of DNA.
  • The transcription bubble can transcribe approximately 50 nucleotides/sec, with newly synthesized RNA intertwining with the DNA template strand for about 8 bp.

RNA Polymerase Proofreading

  • RNA polymerases exhibit higher error rates compared to DNA replication.
  • This increase in errors is acceptable as these errors are not passed on to progeny.
  • Proofreading Process: Upon encountering incorrect nucleotides, RNA polymerase pauses, backtracks, and utilizes a metal ion and water for hydrolytic cleavage of incorrect phosphodiester bonds.

Transcription Termination

  • Termination is highly regulated:
    • Halting of phosphodiester linkages.
    • Dissociation of RNA-DNA hybrids.
    • Reannealing of the melted DNA region.
    • Release of RNA polymerase from the DNA template.
  • Signals for termination involve specific sequences in the DNA as well as RNA products carrying out termination processes.

Termination Signals

  • Palindromic (inverted repeat) GC-rich regions serve as simple termination signals, followed by sequences of T residues.
  • The resultant RNA activity yields complementary structures that facilitate termination:
    • A critical feature for intrinsic termination includes a stem-loop structure with at least 4 uracil residues.

Rho Protein Functionality

  • Rho protein can assist in terminating transcription for certain genes, known as protein-dependent termination.
  • Rho binds segments of the newly synthesized RNA strand and aids in detachment from the DNA template and RNA polymerase.
  • Commonalities in both intrinsic and dependent terminators affirm that signals exist within the RNA strand itself.

tRNA and rRNA Precursors

  • Products of RNA synthesis often undergo additional processing:
    • RNA molecules must be cleaved or chemically modified to achieve functional states.
    • Specific enzymes known as ribonucleases facilitate cleavage from RNA precursors.

Additional Processing Steps

  • tRNA processing involves nucleotide additions at termini, while modifications of bases and ribose units enhance configurational and functional diversity.

The lac Operon

  • The lac operon exemplifies regulation of bacterial gene expression.
  • It governs enzymes that metabolize lactose, particularly β-galactosidase, which catalyzes the breakdown of lactose into galactose and glucose.

Regulation Observations

  • E. coli devoid of lactose has minimal β-galactosidase molecules (<10).
  • In presence of lactose, levels surge into the thousands, illustrating coordinated upregulation of metabolic enzymes responding to environmental shifts.

Operon Structure

  • Operons typically encompass:
    • A regulator gene: Encodes a repressor protein that links to the operator site inhibiting transcription without lactose.
    • An operator site: Regulates the flow of genetic information.
    • A promoter site: Directs RNA polymerase toward correct transcription initiation points.

Ligand Induced Structural Changes

  • Upon the binding of ligands, an inducer molecule interacts with the lac repressor:
    • Induces structural changes reducing the repressor's affinity towards operator DNA, allowing transcription to ensue post-repressor removal.

Transcription Activation Models

  • The lac repressor acts exemplarily in negative control mechanisms.
  • Positive control requires regulatory protein that promotes transcription when glucose availability is low, steering E. coli towards lactose metabolism as fuel.

RNA Processing Overview

  • Nearly all mRNA precursors in higher eukaryotes engage in splicing, which involves:
    • Extraction of introns (noncoding segments) from precursor mRNA genes, leaving exons that remain interconnected in the final mRNA product.
  • Spliced mRNA can be significantly smaller than precursor sizes, while other forms of RNA like tRNA/rRNA undergo necessary extensive processing as well.

Mature rRNA Generation

  • RNA polymerase I catalyzes the transcription of a precursor encoding three ribosomal components:
    • These components require cleavage post extensive modification by small nucleolar ribonucleoproteins (snoRNPs), occurring within the nucleolus.

Transfer RNA Processing

  • tRNA precursors are synthesized by RNA polymerase III.
    • Mature tRNA production necessitates:
    • The removal of a 14-nucleotide intron.
    • Cleavage of a 5' leader.
    • Removal of a 3' trailer.
    • Additional base modifications are customary.

Messenger RNA Processing

  • mRNA processing occurs elaborately:
    • 5' caps formation (triphosphate linkage to 7-methylguanylate) improves stability and translational efficiency.
    • Polyadenylation at the mRNA 3' end adds ~250 adenylate residues post-transcription, following a cleavage signaled by AAUAAA sequence recognition.

mRNA Splicing Dynamics

  • Pre-mRNA splicing necessitates recognition sequences at 5' and 3' splice sites (GU and AG, respectively) and a branch site located upstream.
  • The spliceosome, composed of specific small nuclear RNAs (snRNAs) and over 300 proteins called snRNPs plays a quintessential role in splice site alignment and catalysis.

Spliceosome Assembly

  • The assembly of the spliceosome culminates in two transesterification steps leading to mature mRNA and lariat intron formation.
  • Key features driving the splicing process comprise:
    • Direct alignment aided by snRNPs.
    • Helicase activities powered by ATP to unwind RNA duplexes during catalysis.

Alternative Splicing

  • A mechanism for generating protein diversity, with 70% of human protein-coding genes being alternatively spliced.
  • Variations allow different protein forms necessary for:
    • Tissue specificity
    • Developmental stages
    • Signaling pathways

Diseases Linked to Alternative Splicing

  • Selected human diseases attributed to defects in alternative splicing include:
    • Acute intermittent porphyria (Porphobilinogen deaminase)
    • Breast and ovarian cancer (BRCA1)
    • Cystic fibrosis (CFTR)
    • Frontotemporal dementia (T protein)
    • Hemophilia A (Factor VIII)
    • Lesch-Nyhan syndrome (HGPRT deficiency)
    • Leigh encephalomyelopathy (Pyruvate dehydrogenase E1a)
    • Severe combined immunodeficiency (Adenosine deaminase)
    • Spinal muscle atrophy (SMN1 or SMN2)

RNA Editing and Catalytic Function

  • RNA editing: A process for diverse protein generation post-transcription via specific nucleotide alterations.
  • Certain RNAs exhibit catalytic functions known as ribozymes, facilitating self-splicing in various organisms except vertebrates.

Summary of Key Concepts

  • RNA is synthesized via RNA polymerases in three distinct stages.
  • The regulation of the lac operon provides insight into bacterial gene expression control mechanisms.
  • rRNA and tRNA undergo substantial processing, whereas mRNA is modified and spliced extensively to form mature molecules.
  • RNA can specifically demonstrate catalytic activity across organisms except in vertebrates, enriching diversity in cellular biochemistry.