Prokaryotic and Eukaryotic Transcription: Mechanisms and Regulation

Prokaryotic and Eukaryotic Transcription

Overview

Transcription is the process by which genetic information from DNA is copied into RNA. This process is fundamental to gene expression and is tightly regulated in both prokaryotic and eukaryotic cells. The following notes summarize the mechanisms, regulation, and key differences in transcription between prokaryotes and eukaryotes.

Sigma Factor Switching in Prokaryotes

Role of Sigma Factors

Sigma factors are proteins that associate with the core RNA polymerase enzyme to confer promoter specificity. Different sigma factors are expressed under various conditions, allowing the cell to regulate which genes are transcribed.

• Sigma factors compete for available core RNA polymerase enzymes.

• The holoenzyme (core enzyme + sigma factor) uses a specific sigma factor depending on the cell's needs.

• Expression levels of sigma factors determine which genes are transcribed.

• Example: Under normal growth, σ70 is dominant. Under heat shock, σ32 increases, switching transcription to heat-shock genes.

Promoter Recognition by Sigma Factors

Sigma factors recognize specific DNA sequences at the promoter region, typically at the -35 and -10 positions relative to the transcription start site.

Sigma Factor	-35 Sequence	Separation	-10 Sequence
σ70	TTGACA	16–18 bp	TATAAT
σ32	CCCTTGAA	13–15 bp	CCGGAATT
σ54	CTGGNA	6 bp	TTGCA
σF	CTAAA	5 bp	GCGGAATA
σH	AGGANPru	11–12 bp	GCTGAATCA

Stages of Prokaryotic Transcription

Main Steps

Transcription in prokaryotes occurs in three main stages:

• Initiation: Formation of the transcription bubble and assembly of the transcription machinery.

• Elongation: RNA polymerase synthesizes the RNA strand.

• Termination: RNA polymerase and RNA are released from the DNA template.

Initiation is almost always the rate-limiting step and is subject to regulation to conserve energy.

Initiation Details

• Closed Complex Formation: Duplex DNA interacts with the sigma factor, typically from -55 to +1.

• DNA remains double-stranded (closed).

• RNA polymerase binding is reversible at this stage.

• Open Complex Formation: DNA unwinds at the -10 region (TATA box), allowing the template strand to enter the enzyme's active site. This step is irreversible.

Abortive Initiation: RNA polymerase may synthesize and release short RNA chains before successfully escaping the promoter to begin elongation. This is a rate-limiting step and occurs in both prokaryotes and eukaryotes.

Transcription Bubble and Complexes

Transcription Bubble

The transcription bubble is a region of unwound DNA where RNA synthesis occurs. It moves along the DNA as transcription proceeds and closes after the polymerase passes.

Ternary Complex

• Contains RNA, DNA, and the enzyme (RNA polymerase).

• Forms at the +1 site, where RNA synthesis begins.

• RNA polymerase may be stuck at the promoter, leading to cycles of abortive initiation until promoter escape occurs.

Transcription Termination in Prokaryotes

Types of Terminators

Termination signals in prokaryotes are found at the end of genes and are recognized by RNA polymerase. There are two main types:

• Intrinsic (Rho-independent) Terminators: Do not require additional proteins. Characterized by a GC-rich stem-loop followed by a U-rich region in the RNA.

• Rho-dependent Terminators: Require the Rho protein, a helicase that binds to the nascent RNA and causes release of the RNA from the DNA-RNA hybrid.

Intrinsic Terminator Features

• GC-rich stem-loop structure near the end of the RNA.

• U-rich region downstream of the stem-loop.

• Weak A-U base pairing leads to destabilization and termination.

Rho-dependent Termination

• Rho protein binds to a specific sequence on the RNA.

• Rho uses helicase activity to unwind the RNA-DNA hybrid, releasing the RNA.

Comparison: Prokaryotic vs. Eukaryotic Transcription

Key Differences

• Chromatin Structure: Eukaryotic DNA is packaged into chromatin, affecting accessibility for transcription.

• Promoter Recognition: Prokaryotes use sigma factors; eukaryotes use general transcription factors (GTFs) and more complex promoter elements.

• RNA Polymerases: Eukaryotes have three main RNA polymerases (I, II, III), each transcribing different types of RNA.

• Termination: Eukaryotic termination involves cleavage and polyadenylation signals rather than simple terminator sequences.

Table: Comparison of Prokaryotic and Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Promoter Recognition	Sigma factors	General transcription factors (GTFs)
Chromatin	No chromatin	Chromatin structure regulates access
RNA Polymerases	One main RNA polymerase	Three (Pol I, II, III)
Termination	Intrinsic/Rho-dependent	Cleavage and polyadenylation signals

Summary

Transcription is a highly regulated process essential for gene expression. Prokaryotes utilize sigma factors for promoter specificity and have simpler termination mechanisms, while eukaryotes rely on complex chromatin structure, multiple RNA polymerases, and intricate promoter and termination signals. Understanding these differences is crucial for studying gene regulation and expression in various organisms.