Regulation of gene expression

Prokaryotic Regulation - Operon Operators

• Operon: A cluster of genes transcribed as a single mRNA, under the control of one promoter.
• Components:
  • Promoter: Site where RNA polymerase binds to initiate transcription.
  • Operator: DNA segment where a repressor protein binds, blocking transcription.
  • Regulator Gene: Codes for the repressor protein (e.g., lacI, trpR); may be located far from the operon.

Lac Operon (Inducible)

• Function: Catabolizes lactose in E. coli when glucose is absent.
• Regulation:
  • Negative Control: Repressor protein (lacI product) is constitutively active, binding the operator to block transcription. Allolactose (an inducer) binds the repressor, causing its release.
  • Positive Control: Low glucose leads to ↑cAMP, which binds to Catabolite Activator Protein (CAP). The cAMP-CAP complex binds the promoter, significantly boosting transcription.

⭐ The lac operon is a classic example of dual control, requiring both the removal of a negative regulator (repressor) and the presence of a positive regulator (cAMP-CAP complex) for maximal expression.

Trp Operon (Repressible)

• Function: Synthesizes tryptophan when it is unavailable in the environment.
• Regulation: Repressor protein is constitutively inactive. Tryptophan acts as a corepressor; when present, it binds and activates the repressor, which then binds the operator to halt transcription.

Eukaryotic Regulation I - Chromatin Control

• Chromatin: DNA wraps around histone octamers (2x H2A, H2B, H3, H4) to form nucleosomes, creating a "beads on a string" structure. The accessibility of this DNA is a primary site of gene regulation.
• Chromatin States:
  • Euchromatin: Less condensed, accessible to transcription factors. Transcriptionally active. (Eu = true)
  • Heterochromatin: Highly condensed, inaccessible. Transcriptionally inactive. (Hetero = different)
• Epigenetic Modifications: Reversible, heritable changes that don't alter the DNA sequence.
  • Histone Acetylation:
    • Histone Acetyltransferases (HATs) add acetyl groups to lysine residues on histone tails.
    • Neutralizes lysine's positive charge, relaxing DNA coiling → ↑ transcription.
    • 📌 Histone Acetylation makes chromatin Active.
  • Histone Deacetylation:
    • Histone Deacetylases (HDACs) remove acetyl groups, tightening the coil → ↓ transcription.
  • DNA Methylation:
    • DNA Methyltransferases (DNMTs) add methyl groups to cytosine bases in CpG islands (often in promoters), leading to gene silencing.

⭐ In many cancers, tumor suppressor genes are silenced via hypermethylation of their promoter regions.

Eukaryotic Regulation II - Transcriptional & Post-Transcriptional

• Transcriptional Control: Fine-tuning gene expression.
  • Enhancers/Silencers: DNA sequences that bind activator/repressor proteins. Can be located far from the promoter, influencing transcription rates.
  • Transcription Factors: Proteins that bind to DNA regulatory elements, modulating the assembly of the transcription machinery.
• Post-Transcriptional Modification: Processing of pre-mRNA in the nucleus before export.

• RNA Interference (RNAi):
  • MicroRNA (miRNA) & small interfering RNA (siRNA) bind to complementary mRNA, leading to degradation or translational repression.

⭐ Alternative splicing allows a single gene to encode multiple distinct protein variants by selectively removing different introns and exons. This dramatically expands proteomic diversity from a limited number of genes.

High‑Yield Points - ⚡ Biggest Takeaways

• The lac operon is an inducible system for catabolism, activated by allolactose when glucose is absent.
• The trp operon is a repressible system for anabolism, turned off by high levels of tryptophan.
• Eukaryotic regulation hinges on transcription factors, enhancers, and silencers.
• Histone acetylation generally activates transcription by uncoiling DNA.
• DNA methylation at CpG islands typically leads to gene silencing.