Transcription and RNA Processing: Mechanisms and Regulation in Bacteria and Eukaryotes

Transcription: The Central Dogma of Molecular Biology

Overview of Genetic Information Flow

The central dogma describes the flow of genetic information from DNA to RNA to protein. Transcription is the process by which RNA is synthesized from a DNA template, and translation is the subsequent process where proteins are synthesized from RNA.

• Transcription: DNA is used as a template to produce RNA.

• Translation: RNA is decoded to synthesize proteins.

• Replication: DNA is copied to produce more DNA.

• Coding information: mRNA carries coding information in codons, which are read by the translation machinery.

Mechanism of Transcription

Transcription Process and Enzymes

Transcription is catalyzed by RNA polymerase, which synthesizes RNA using a DNA template. Unlike DNA polymerase, RNA polymerase does not require a primer and uses ribonucleotides as substrates.

• RNA Polymerase: Enzyme responsible for RNA synthesis.

• Substrate requirements: Uses ribonucleotides (ATP, UTP, GTP, CTP).

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

• No proofreading: RNA polymerase lacks proofreading activity.

Stages of Transcription

Transcription occurs in three main stages: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter region of DNA, forming a closed complex. The DNA unwinds to form an open complex, and RNA synthesis begins.

• Elongation: RNA polymerase moves along the template strand, synthesizing RNA in the 5' to 3' direction.

• Termination: RNA polymerase releases the newly synthesized RNA and dissociates from the DNA template.

Transcription in Bacteria

Promoters and Consensus Sequences

Bacterial promoters contain conserved DNA sequences that facilitate RNA polymerase binding and transcription initiation. The most important regions are the -35 and -10 sequences, also known as the Pribnow box.

• Consensus sequences: TTGACA (-35) and TATAAT (-10).

• Promoter location: Upstream of the transcription start site (+1).

• Numbering: Bases upstream are negative; downstream are positive.

RNA Polymerase Structure and Function

Bacterial RNA polymerase consists of a core enzyme and a sigma factor. The sigma factor is essential for promoter recognition and initiation.

• Core enzyme: Five subunits (α2ββ'ω).

• Sigma factor: One subunit (σ), responsible for promoter recognition.

Initiation, Elongation, and Termination in Bacteria

Transcription initiation relies on the ability of RNA polymerase to bind to the promoter. Elongation involves the synthesis of RNA, and termination can occur via two mechanisms: rho-dependent and rho-independent.

• Rho-dependent termination: Requires the rho protein, which binds to the RNA and causes dissociation.

• Rho-independent termination: Involves a stem-loop structure in the RNA followed by a uracil-rich sequence, causing RNA polymerase to pause and dissociate.

Transcription in Eukaryotes

Complexity and Regulation

Eukaryotic transcription is more complex due to larger genomes, cellular complexity, and multicellularity. Regulation is achieved through multiple RNA polymerases and various cis- and trans-acting elements.

• RNA polymerase I: Transcribes rRNA genes (except 5S rRNA).

• RNA polymerase II: Transcribes protein-encoding genes and some snRNA genes.

• RNA polymerase III: Transcribes tRNA genes and 5S rRNA gene.

Promoters and Regulatory Elements

Eukaryotic promoters include the core promoter (TATA box), proximal-promoter elements, enhancers, and silencers. Transcription factors (activators and repressors) bind to these elements to regulate gene expression.

• Core promoter: Contains the TATA box and transcriptional start site.

• Enhancers: Increase transcription rates.

• Silencers: Decrease transcription rates.

• Transcription factors: Facilitate RNA polymerase binding and regulate transcription.

General Transcription Factors

General transcription factors (GTFs) are essential for the formation of the basal transcription apparatus and initiation of transcription by RNA polymerase II.

GTF	Function
TFIID	Binds to the TATA box and recruits other GTFs
TFIIB	Promotes RNA polymerase II binding
TFIIF	Stabilizes RNA polymerase II binding
TFIIE	Helicase activity, forms open complex
TFIIH	Helicase and kinase activity, phosphorylates RNA polymerase II

Post-Transcriptional RNA Processing in Eukaryotes

mRNA Modification

Eukaryotic mRNA undergoes several modifications before leaving the nucleus, including capping, polyadenylation, and splicing.

• 5' Cap: Addition of a 7-methyl guanosine cap at the 5' end, important for stability and translation.

• Poly-A Tail: Addition of a string of adenine nucleotides at the 3' end, important for mRNA stability.

• Splicing: Removal of introns and joining of exons by the spliceosome.

Splicing and the Spliceosome

Splicing is carried out by the spliceosome, a complex of snRNPs that recognizes intron-exon boundaries and catalyzes intron removal.

• Introns: Non-coding regions removed from pre-mRNA.

• Exons: Coding regions joined to form mature mRNA.

• Alternative splicing: Allows production of different mRNAs from the same gene.

RNA Editing

RNA editing is a posttranscriptional process where the nucleotide sequence of an RNA molecule is altered by insertion, deletion, or substitution. This is prevalent in mitochondrial and chloroplast RNAs in plants.

Comparison of Transcription and RNA Modification in Bacteria and Eukaryotes

Key Differences

Bacterial and eukaryotic transcription differ in promoter structure, regulatory mechanisms, RNA polymerase types, and post-transcriptional modifications.

Component	Bacteria	Eukaryotes
Promoter	-10 and -35 sequences	TATA box and transcriptional start site
Initiation	Sigma factor required	General transcription factors required
Elongation	Simple	Complex, regulated
Splicing	Rare	Common, requires spliceosome
Capping	Does not occur	Addition of 7-methyl guanosine cap
Polyadenylation	Does not occur	Addition of poly-A tail

Summary

Transcription is a fundamental process in genetics, converting DNA information into RNA, which is then translated into proteins. The mechanisms and regulation of transcription differ between bacteria and eukaryotes, with eukaryotes exhibiting greater complexity and post-transcriptional modifications. Understanding these processes is essential for studying gene expression and regulation in molecular genetics.