Transcription and Regulation of Gene Expression in Bacteria and Eukaryotes

Gene Expression and Transcription

Overview of Gene Expression

Gene expression is the process by which information from a gene is used to synthesize functional gene products, such as proteins. This process involves transcription (copying DNA to RNA) and translation (RNA to protein). Regulation of gene expression ensures that genes are expressed at the right time, place, and amount.

• Transcription: The synthesis of RNA from a DNA template.

• Gene Expression Regulation: Mechanisms that control the timing, location, and amount of gene product produced.

RNA Polymerase and Transcription Initiation

RNA Polymerase (Bacterial Focus)

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template. In bacteria, a single RNA polymerase carries out transcription, while eukaryotes have multiple types.

• Polymerizes RNA in the 5' to 3' direction.

• Does not require a primer to initiate synthesis.

• Bacterial RNA polymerase requires a sigma factor to recognize promoters and initiate transcription.

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

Promoter Recognition and Sigma Factor (Bacteria)

Transcription begins when RNA polymerase binds to a specific DNA sequence called the promoter. In bacteria, the sigma factor guides RNA polymerase to the promoter.

• Promoter: DNA sequence upstream of the gene; contains conserved regions such as the -10 (TATAAT) and -35 (TTGACA) boxes.

• Sigma Factor: Protein that binds to RNA polymerase, forming a holoenzyme, and directs it to the promoter.

• Once bound, sigma dissociates, and RNA polymerase begins RNA synthesis.

Process of Transcription (Bacteria)

1. Initiation

During initiation, RNA polymerase binds to the promoter, unwinds the DNA, and begins RNA synthesis.

• RNA polymerase opens the DNA helix, creating a transcription bubble.

• The template strand is threaded through the active site of the enzyme.

• Ribonucleoside triphosphates (NTPs) enter and pair with complementary DNA bases.

• Polymerization begins at the +1 site (start site).

2. Elongation

RNA polymerase moves along the DNA template, synthesizing RNA in the 5' to 3' direction.

• Nucleotides are added to the 3' end of the growing RNA molecule.

• The enzyme continues elongation until it reaches a termination signal.

3. Termination

Termination occurs when RNA polymerase encounters a specific DNA sequence (termination signal) that causes it to release the newly synthesized RNA and detach from the DNA.

• Termination signal codes for RNA that forms a hairpin structure, disrupting the transcription complex.

• RNA polymerase separates from the RNA transcript.

Differences Between Bacterial and Eukaryotic Transcription

• Number of RNA Polymerases: Bacteria have one; eukaryotes have three.

• Promoter Structure: Eukaryotic promoters are more complex (e.g., TATA box).

• Initiation Factors: Eukaryotes use general transcription factors instead of sigma factors.

• Termination: Eukaryotes use a poly(A) signal; bacteria use a terminator sequence.

• Location: Transcription occurs in the nucleus in eukaryotes and in the cytoplasm in bacteria.

Regulation of Gene Expression

Levels of Regulation

Gene expression can be regulated at multiple steps, but the greatest control is often at the level of transcription initiation.

• Transcriptional Regulation: Determines whether and how much mRNA is produced from a gene.

• Post-transcriptional Regulation: Controls mRNA processing, stability, and translation.

• Post-translational Regulation: Modifies proteins after synthesis, affecting their activity.

Mechanisms of Transcriptional Regulation

• Transcription Factors (TFs): Proteins that bind to specific DNA sequences to activate or repress transcription.

• Repressors: Proteins that inhibit transcription by blocking RNA polymerase binding or activity.

• Activators: Proteins that enhance transcription by facilitating RNA polymerase binding or activity.

Model System: Regulation of the lac Operon in E. coli

Organization of the lac Operon

The lac operon is a group of genes involved in lactose metabolism in Escherichia coli. These genes are transcribed together as a single mRNA and regulated as a unit.

• lacZ: Encodes β-galactosidase, which breaks down lactose into glucose and galactose.

• lacY: Encodes galactoside permease, which transports lactose into the cell.

• lacA: Encodes transacetylase (function less central to lactose metabolism).

Negative Control of the lac Operon

Negative control involves a repressor protein that binds to the operator region of the operon, preventing transcription.

• No lactose present: The repressor binds to the operator, blocking RNA polymerase and preventing transcription.

• Lactose present: Lactose (the inducer) binds to the repressor, causing an allosteric change that releases the repressor from the operator, allowing transcription.

Positive Control of the lac Operon

Positive control involves an activator protein (CAP) that enhances transcription when glucose is low.

• CAP (catabolite activator protein): Binds to the promoter in the presence of cAMP (produced when glucose is low), increasing transcription of the lac operon.

• Glucose present: Low cAMP levels prevent CAP binding, reducing transcription even if lactose is present.

Summary Table: Regulation of the lac Operon

Condition	Repressor	CAP	Transcription
No lactose, high glucose	Bound to operator	Inactive	OFF
Lactose present, high glucose	Released from operator	Inactive	LOW
Lactose present, low glucose	Released from operator	Active (bound to DNA)	HIGH

Key Terms and Concepts

• Transcription: Synthesis of RNA from a DNA template.

• RNA Polymerase: Enzyme that synthesizes RNA.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA region where regulatory proteins bind to control transcription of an operon.

• Repressor: Protein that inhibits gene expression by binding to the operator.

• Activator: Protein that increases gene expression by facilitating RNA polymerase binding.

• cAMP: Cyclic AMP, a signaling molecule that activates CAP.

Equations and Diagrams

• Transcription Reaction:

$\text{(DNA template)} + NTPs \xrightarrow{\text{RNA polymerase}} \text{RNA} + PP_i$

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info:

• In eukaryotes, transcription and translation are separated by the nuclear envelope, allowing for additional regulation (e.g., RNA processing).

• Transcription factors can act as activators or repressors, and their activity is often regulated by cell signaling pathways.