RNA Processing, Regulatory RNAs, and RNA Interference

Processing of Ribosomal and Transfer RNA

Ribosomal RNA (rRNA) Processing

Ribosomal RNA is transcribed as a large precursor molecule and then processed to yield mature rRNAs, which are essential components of ribosomes. The processing steps and the types of rRNAs differ between prokaryotes and eukaryotes.

• Prokaryotic rRNAs: 16S (small subunit), 23S and 5S (large subunit)

• Eukaryotic rRNAs: 18S (small subunit), 5.8S, 28S, and 5S (large subunit)

• Svedberg units (S): A measure of sedimentation rate during centrifugation, reflecting size and shape.

Example: In eukaryotes, a single pre-rRNA transcript is processed to yield 18S, 5.8S, and 28S rRNAs, while 5S rRNA is transcribed separately.

Transfer RNA (tRNA) Processing

tRNAs are also transcribed as precursor molecules and undergo extensive processing, including cleavage, splicing, and base modification, to become functional.

• Cleavage: Removal of 5' leader and 3' trailer sequences.

• Base Modifications: Over 80 different modifications can occur, such as methylation and pseudouridylation.

Example: Pseudouridine (Ψ) is a common tRNA modification, where uridine is isomerized to pseudouridine, enhancing tRNA stability and function.

tRNA Modifications in Human tRNAs

Types and Functions of tRNA Modifications

Human tRNAs contain numerous modified nucleotides, which are critical for proper folding, stability, and accurate translation.

• Pseudouridine (Ψ): Formed by isomerization of uridine; increases tRNA stability.

• Methylation (e.g., m1A, m5C): Protects tRNA from degradation and affects codon recognition.

• Other modifications: Inosine (I), dihydrouridine (D), and queuosine (Q) are also common.

Example: The cloverleaf structure of tRNA shows the positions of various modifications, which are essential for tRNA's role in translation.

Central Dogma and RNA Transcription

Overview of the Central Dogma

The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• DNA Replication: Copying of DNA to produce identical DNA molecules.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of proteins using mRNA as a template.

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

RNA Transcription Steps

• Initiation: RNA polymerase binds to promoter region.

• Elongation: RNA strand is synthesized in the 5' to 3' direction.

• Termination: RNA polymerase releases the completed RNA transcript.

• Processing (in eukaryotes): Includes capping, polyadenylation, and splicing.

Comparison: Prokaryotic transcription occurs in the cytoplasm and is often coupled with translation, while eukaryotic transcription occurs in the nucleus and involves extensive RNA processing.

Regulatory RNAs

Major RNA Molecules and Their Functions

RNA molecules play diverse roles in gene expression and regulation. The following table summarizes the main types and their functions:

RNA Type	Function
Messenger RNA (mRNA)	Encodes the sequence of amino acids in a polypeptide.
Ribosomal RNA (rRNA)	Forms the core of ribosome's structure and catalyzes protein synthesis.
Transfer RNA (tRNA)	Brings amino acids to ribosomes during translation.
Small nuclear RNA (snRNA)	Functions in splicing of pre-mRNA in the nucleus.
MicroRNA (miRNA)	Regulates gene expression by base pairing with mRNAs, leading to their degradation or translational repression.
Small interfering RNA (siRNA)	Regulates gene expression by promoting mRNA degradation; involved in RNA interference.

Additional info: Other regulatory RNAs include long non-coding RNAs (lncRNAs), piwi-interacting RNAs (piRNAs), and small nucleolar RNAs (snoRNAs).

Types of Small RNAs

Classes and Functions

Small RNAs are a diverse group of regulatory molecules that control gene expression at multiple levels.

• MicroRNA (miRNA): Endogenous, ~22 nt, regulates gene expression post-transcriptionally.

• Small interfering RNA (siRNA): Exogenous or endogenous, ~21-23 nt, mediates RNA interference.

• Piwi-interacting RNA (piRNA): ~24-31 nt, silences transposons in germ cells.

• Argonaute protein: Central component of RNA-induced silencing complex (RISC).

• Small nuclear RNA (snRNA): Involved in splicing.

• Small nucleolar RNA (snoRNA): Guides chemical modifications of rRNA.

• tRNA fragment (tRF): Derived from tRNAs, may have regulatory roles.

Example: miRNAs guide RISC to target mRNAs, leading to their silencing.

Discovery of RNA Interference (RNAi)

Mechanism and Significance

RNA interference (RNAi) is a biological process in which double-stranded RNA (dsRNA) induces the silencing of specific genes. Discovered by Andrew Fire and Craig Mello in C. elegans, RNAi has become a powerful tool in genetics research.

• dsRNA triggers gene silencing: Introduction of dsRNA leads to degradation of complementary mRNA.

• Applications: Used to "knock down" gene expression to study gene function.

Example: Injection of dsRNA targeting a specific gene in C. elegans results in loss of gene function and observable phenotypic changes.

Mechanism of RNA Silencing

Steps in RNAi Pathway

• Dicer enzyme: Cleaves dsRNA into small fragments (~21-25 nt).

• RISC complex: Binds one strand (guide strand) of the small RNA.

• Target recognition: Guide strand directs RISC to complementary mRNA.

• Gene silencing: mRNA is cleaved or translation is inhibited.

Equation:

$\text{dsRNA} \xrightarrow{\text{Dicer}} \text{siRNA/miRNA} \xrightarrow{\text{RISC}} \text{mRNA silencing}$

Long Non-coding RNAs (lncRNAs) and X Chromosome Inactivation

Role of lncRNAs

Long non-coding RNAs (lncRNAs) are transcripts longer than 200 nucleotides that do not code for proteins but have regulatory functions.

• Xist lncRNA: Essential for X chromosome inactivation in female mammals.

• Mechanism: Xist RNA coats the X chromosome to be inactivated, recruiting chromatin-modifying complexes that silence gene expression.

• Barr body: The inactivated X chromosome forms a dense structure in the nucleus.

Example: In female mammals, one X chromosome is randomly inactivated in each cell to balance gene dosage between sexes.

RNA-binding Proteins (RBPs)

Functions and Importance

RNA-binding proteins are essential for RNA metabolism, including splicing, transport, stability, and translation.

• Conservation: RBPs are highly conserved and abundant in cells.

• Tissue specificity: Some RBPs are expressed in specific tissues, influencing cell-type-specific gene expression.

Example: The Argonaute family of proteins is central to the function of RISC in RNA silencing.

RNA Isolation and Sequencing

RNA Quality and Integrity

High-quality RNA is essential for downstream applications such as sequencing. The RNA Integrity Number (RIN) is used to assess RNA quality.

• RIN: Scale from 1 (degraded) to 10 (intact); higher values indicate better quality.

RNA Sequencing (RNA-seq)

RNA-seq is a powerful technique for analyzing the transcriptome, including coding and non-coding RNAs.

• Steps: Isolate RNA, generate cDNA, fragment, size select, add adapters, and sequence.

• Applications: Quantifies gene expression, identifies novel transcripts, and detects small RNAs.

Example: RNA-seq can identify and quantify miRNAs and other small RNAs in the 18-30 nt range.