LEC 4: Regulation of Gene Expression I

Question 1

Gene Regulation

Answer 1

Gene expression is controlled by the needs of the cell as it responds to its environment with the goal of increasing in mass and dividing

Question 2

General Organisation of Inducible Genes

Answer 2

Inducible genes are only expressed when induced

- Promotor, Controlling Site, Coding sequence, Terminator

Question 3

Regulation of an Operon

Answer 3

Operon regulation is dependent on the presence of a repressor molecule
- Binding of a repressor prevents transcription by RNA polymerase

Question 4

The Lac Operon

Answer 4

If lactose is E. coli’s sole carbon source, three genes are expressed:

1. LAC-Z: β-galactosidase
2. LAC-Y: Permease
3. LAC-A: Transacetylase

Question 5

Translation of Lac Operon

Answer 5

In Presence of Lactose: Lactose binds with Repressor causing conformational change, repressor unable to bind to regulatory protein before LAC-Z, RNApol binds -> translation

In Absence of Lactose: Repressor does not undergo conformational change, blocks RNApol binding on regulatory region before LAC-Z -> no translation

Question 6

Lac Operon Summary

Answer 6

1. Sole Carbon Source: Lactose
   - Positive control of Lac Op
   - CAP-cAMP complex
   - Binding of CAP-cAMP complex causes the DNA to bend, facilitating protein-protein interactions between CAP and RNA polymerase, and leading to transcription.
2. Sole Carbon Source: Glucose
   - Glucose metabolism greatly reduces cAMP levels in the cell
   - The CAP-cAMP level drops and is insufficient to maintain transcription of the lac genes

Question 7

The Tryptophan (Trp) Operon

Answer 7

Absence of Trp: Conc. of Trp too low to bind to repressor
- Trp biosynthetic genes translate and increase in Trp synthesis

Presence of Trp: Conc. of Trp too high

• Trp binds to repressor and induces a conformational change -> no new Trp synthesized
• More attenuated transcripts as the tryptophan concentration increases = less Trp translated

Question 8

Trp Regulation

Answer 8

Two mechanisms regulate expression of the Trp operon:

1. Repressor/operator interaction (aporepressor and Trp complex)
2. Transcription termination

Question 9

Attenuation (Trp Starved)

Answer 9

Antitermination:

• Starvation or low levels of Trp causes ribosome translating leader transcript to stall at Trp Codon
• Sections 2 and 3 form hairpin

Question 10

Attenuation (Non-Starved)

Answer 10

Non-Starved Termination:

- Section 3 and 4 form hairpin