13.5 Regulating Gene Expression

Gene expression must be carefully regulated so that cells produce the right proteins at the right time and in the right amounts. Regulation occurs at multiple levels and differs significantly between prokaryotes and eukaryotes.

---

Gene Regulation in Prokaryotes — The Operon Model

In prokaryotes, genes that encode enzymes for the same metabolic pathway are often grouped together into a functional unit called an operon.

Components of an Operon

Types of Operons

Operons are classified based on whether they are normally active or inactive:

1. Inducible Operons — The Lac Operon

The lac operon of E. coli controls the metabolism of lactose. It is normally OFF (repressed).

When lactose is ABSENT:

• The repressor protein (coded by the lacI gene) is active.
• The repressor binds to the operator, blocking RNA polymerase.
• Structural genes (lacZ, lacY, lacA) are not transcribed.

When lactose is PRESENT:

• Lactose is converted to allolactose (the true inducer).
• Allolactose binds to the repressor, causing a conformational change.
• The repressor can no longer bind the operator.
• RNA polymerase transcribes the structural genes → enzymes for lactose digestion are produced.

2. Repressible Operons — The Trp Operon

The trp operon controls the biosynthesis of the amino acid tryptophan. It is normally ON (active).

When tryptophan is SCARCE:

• The repressor protein is synthesised in an inactive form.
• RNA polymerase transcribes the structural genes → tryptophan is synthesised.

When tryptophan is ABUNDANT:

• Tryptophan acts as a co-repressor by binding to the inactive repressor.
• The repressor–tryptophan complex undergoes a conformational change and becomes active.
• The active complex binds to the operator, blocking transcription.
• Tryptophan synthesis stops (end-product inhibition at the gene level).

Summary: Inducible vs. Repressible

---

The Lac Operon in Detail

---

The Trp Operon in Detail

---

Gene Regulation in Eukaryotes

Eukaryotic gene regulation is far more complex than in prokaryotes. Regulation can occur at multiple levels:

Levels of Eukaryotic Gene Regulation

1. Chromatin remodelling — Tightly packed heterochromatin is inaccessible to RNA polymerase; genes must be in loosely packed euchromatin to be expressed. Histone acetylation loosens chromatin.

2. Transcriptional control — The most important level. Involves:

   • Promoters — sequences upstream of the gene (e.g., the TATA box) where the transcription initiation complex assembles.
   • Enhancers — distal DNA sequences that, when bound by transcription factors, increase transcription rate (can be thousands of base pairs away from the gene).
   • Silencers — sequences that decrease transcription.
   • Transcription factors — proteins that bind to promoters or enhancers to promote or inhibit RNA polymerase recruitment.

3. Post-transcriptional control — Processing of pre-mRNA:

   • Addition of a 5' cap and 3' poly-A tail (increases mRNA stability)
   • RNA splicing — removal of introns and joining of exons (absent in prokaryotes)
   • Alternative splicing — different exon combinations from one gene can produce multiple proteins

4. Translational control — Regulation of when and how much mRNA is translated.

5. Post-translational control — Modification of proteins after synthesis (e.g., phosphorylation, glycosylation).

Transcription Factors

Transcription factors are regulatory proteins that:

• Bind to specific DNA sequences (promoters or enhancers)
• Interact with RNA polymerase or the transcription initiation complex
• Either activate (activators) or repress (repressors) transcription
• Allow cells with identical DNA to express different genes → differential gene expression

---

Operon Model — Overview