Molecular Biology of Transcription and RNA Processing: Study Notes

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Chapter 8: Molecular Biology of Transcription and RNA Processing

8.1 RNA Transcripts Carry the Messages of Genes

Understanding how genetic information in DNA directs protein synthesis was a major milestone in molecular genetics. RNA, a molecule similar to DNA, plays a central role in this process.

Key Point 1: RNA is present in all cells and is chemically similar to DNA, but its function was initially unclear.
Key Point 2: Cell structure suggests RNA's role in protein synthesis, as DNA is located in the nucleus (in eukaryotes) while protein synthesis occurs in the cytoplasm.
Example: Messenger RNA (mRNA) acts as an intermediary, carrying genetic information from DNA to ribosomes for protein synthesis.

The Four RNA Ribonucleotides

RNA is composed of four ribonucleotides, each consisting of a ribose sugar, a phosphate group, and a nitrogenous base.

Purine nucleotides: Adenosine (AMP), Guanosine (GMP)
Pyrimidine nucleotides: Uridine (UMP), Cytidine (CMP)
Key Point: RNA uses uracil (U) instead of thymine (T) found in DNA.

RNA Assembly and Structure

RNA strands are assembled by the formation of phosphodiester bonds between adjacent nucleotides, creating a sugar-phosphate backbone similar to DNA.

Key Point: RNA is synthesized from a DNA template using complementary base pairing: A with U, C with G.
Equation:

Categories of RNA

There are several major categories of RNA, each with distinct functions:

Messenger RNA (mRNA): Produced by protein-coding genes; serves as a template for protein synthesis.
Ribosomal RNA (rRNA): Combines with proteins to form ribosomes, the site of protein synthesis.
Transfer RNA (tRNA): Binds amino acids and delivers them to the ribosome during translation.

Functional RNAs

Functional RNAs do not encode proteins but perform essential cellular roles.

rRNA: Structural and catalytic component of ribosomes.
tRNA: Adapter molecule in translation, matching amino acids to codons in mRNA.

Additional Functional RNAs

Telomerase RNA: Template for telomere sequence synthesis.
Small nuclear RNA (snRNA): Involved in mRNA processing (splicing).
Micro RNA (miRNA) and small interfering RNA (siRNA): Regulate gene expression post-transcriptionally.

Table 8.1 Selected Noncoding RNAs and Their Functions

Type of ncRNA	Description of Function
Telomerase RNA component (TERC)	Template for telomere sequence synthesis during DNA replication.
X-inactivation specific transcript	Inactivates one X chromosome in female mammals.
Ribonuclease P RNA (RNaseP)	Processes tRNA precursors.
Small nuclear RNA (snRNA)	Found in nucleus; involved in splicing.
Transfer RNA (tRNA)	Carries amino acids to ribosome.
Small interfering RNA (siRNA)	Regulates stability and translation of mRNA.
microRNA (miRNA)	Regulates gene expression by base pairing with mRNA.

8.2 Bacterial Transcription Is a Four-Stage Process

Transcription in bacteria is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase. It occurs in four stages:

Promoter recognition
Transcription initiation
Chain elongation
Chain termination

DNA Strand Identification for Transcription

Template strand: Used by RNA polymerase to synthesize complementary RNA.
Coding strand (nontemplate): Sequence matches the RNA (except T is replaced by U).

Gene Structure

Promoter: Upstream of transcription start (+1 nucleotide); controls RNA polymerase access.
Coding region: Contains information for protein synthesis.
Termination region: Downstream of coding region; signals end of transcription.

Bacterial RNA Polymerase

Single RNA polymerase: Transcribes all RNA types in E. coli.
Rifampicin: Antibiotic that inhibits RNA synthesis by blocking the first phosphodiester bond formation.

RNA Polymerase Composition

Core enzyme: Composed of two α, two β, and one ω subunit.
Sigma (σ) subunit: Required for promoter recognition; forms the holoenzyme when bound to the core.
Alternative sigma subunits: Allow recognition of different promoter sequences.

Table 8.2 Escherichia coli RNA Polymerase Sigma Subunits

Subunit	Molecular Weight (kD)	Consensus Sequence -35	Consensus Sequence -10	Function
σ28	28	TAAA	GCCGATAA	Flagellar synthesis and chemotaxis
σ32	32	CTTGAA	CCCCATTA	Heat shock genes
σ54	54	CTGGPyAPyPu*	TTGCA	Nitrogen metabolism
σ70	70	TTGACA	TATAAT	Housekeeping genes

*Py = pyrimidine; Pu = purine

Bacterial Promoters and Consensus Sequences

Promoter: Double-stranded DNA sequence where RNA polymerase binds.
Consensus sequences:
- -10 (Pribnow box): 5'-TATAAT-3'
- -35: 5'-TTGACA-3'

Transcription Initiation

Step 1: Holoenzyme loosely attaches to promoter, then tightly binds to form the closed promoter complex.
Step 2: Holoenzyme unwinds ~18 bp of DNA at -10 to form the open promoter complex.
Step 3: RNA synthesis begins at the +1 site.

Transcription Elongation and Termination

Elongation: Sigma subunit dissociates after 8-10 nucleotides; core enzyme continues RNA synthesis.
Termination: RNA polymerase releases the RNA transcript and dissociates from DNA.

Transcription Termination Mechanisms

Intrinsic termination: Relies on inverted repeat sequences followed by a string of adenines, forming a hairpin structure in mRNA.
Rho-dependent termination: Requires rho protein and a rut site; rho protein binds and releases the mRNA from RNA polymerase.

Intrinsic Termination

Termination sequence: Inverted repeat followed by adenines.
Hairpin structure: mRNA forms a stem-loop, destabilizing the RNA-DNA hybrid and causing release.

Rho-Dependent Termination

Rho protein: Binds to rut site on mRNA, moves toward RNA polymerase, and catalyzes release of mRNA and polymerase from DNA.

Additional info: These notes cover the molecular basis of transcription and RNA processing, including the structure and function of RNA, transcription mechanisms in bacteria, and the roles of various RNA types. The tables provide a comparative overview of noncoding RNAs and sigma subunits, which are essential for understanding gene regulation and expression in prokaryotes.