Lecture 31: Gene Regulation

Question 1

2 Types of gene expression

Answer 1

Constitutive genes:

• Always expressed
• Housekeeping genes

Regulated genes:
-Only need to be expressed at certain times

Question 2

Where does Gene regulation take place?

Answer 2

Takes place in:

1) Transcription
2) mRNA processing
3) Translation
4) Regulation of protein half life

-majority of regulation in bacteria takes
place at the transcriptional level

Question 3

What regulates genes in (Prokaryotes)

Answer 3

Operator = upstream of promoter

-A binding site for specific proteins that help to regulate gene expression

Question 4

Types of regulatory proteins

Answer 4

1) Activator proteins = bind to or near an operator region and allow RNA polymerase to initiate transcription
2) Repressors = bind to an operator region and prevent RNA polymerase from initiating transcription

Question 5

If lactose is present E. coli produce what 3 products?

Answer 5

1) LacZ - B-galactosidase:
(lactose > glucose +galactose)
(lactose > allolactose)

2) LacY – Permease
(active transport of lactose across cell membrane)

3) LacA – Transacetylase
(galactose > acetylegalactose)

• All three genes share the same promoter (lacP), the same operator (lacO) and are transcribed as a single mRNA (polycistronic mRNA)
• lacZ, lacY and lacA should only be expressed when lactose is present in the cell (lacI gene encodes a repressor protein when lactose isn’t present)

Question 6

Induction and Repression of lac operon

Answer 6

Lactose present = lac operon turned on = inducible = RNA initiates transcription at promoter

Lactose absent = lac operon turned off = Repressor (LacI) bound to operator, RNA polymerase cannot initiate
transcription at the promoter

Question 7

What happens if there is a repressor and lactose is added

Answer 7

1) There will be formation of the isomer, allolactose, which will bind to the repressor
2) Allolactose-bound repressor undergoes a conformational change and dissociates from the operator sequence
3) Lactose system = turned on
4) RNA polymerase is then free to initiate transcription

Question 8

Mutations affecting Lac and Lac operon

Answer 8

Lac:
1) lacZ-, lacY-, lacA-: Structural gene mutations = non-functional proteins

2) lacP-: Non-functional promoter = RNA pol cannot bind so genes will not be expressed

3) lacOC: Non-functional operator = repressor cannot bind.
Since the system cannot be shut off, this is a
constitutive mutation

Lac Operon:
1) lacI-: Non-functional repressor = unable to bind the operator to shut off transcription

2) lacIS: Super-repressor = unable to dissociate from operator, System always off

Question 9

Regulation of Lac

Kinds of control?

Answer 9

Repressor = negative control

Lac operon on = positive control = increases efficiency of transcription

Question 10

Glucose and cAMP

Answer 10

no glucose = cAMP production
= cAMP binds the activator protein CRP (cAMP receptor protein) or CAP (catabolite activator protein), which can then bind lacP to help activate TRANSCRIPTION

Glucose present = no cAMP production
= CRP is inactive = RNA pol cannot EFFICIENTLY initiate transcription

Question 11

When will Lac Operon only be transcribed

Answer 11

1) Lactose present

2) Glucose absent

Question 12

Gene regulation Eukaryotes:

Transcriptional control

Answer 12

Transcriptional control is more complex in eukaryotes because:

1) nuclear compartmentalization
2) chromatin remodelling,
3) recruitment of transcription machinery,
4) transcription initiation,
5) elongation
6) termination

Question 13

Cis Vs. Trans Regulatory Elements

Answer 13

Cis:
-DNA sites where proteins and trans regulatory elements will bind

1) Basal promoter sequence (Bind the general transcription factor protein which are associated w RNA pol)
2) Proximal Control regions (Bind transcription factor proteins, found near the promoter)
3) Enhancer Sequence (Found far away from promoter)

Trans:
-Proteins and transcription factors which will bind to the DNA CIS elements

Question 14

E. Coli’s favorite Carbon source?

Answer 14

Glucose

• Presence of glucose [preferred] no need to express genes in the lac operon)
• Positive control of lac operon (genetic expression occurs only if a regulator molecule directly stimulates RNA production)

Question 15

Is Lac operon on or off? Activators/repressors?

1) Lactose present + glucose present
2) Lactose present + glucose absent
3) Lactose absent + glucose present
4) Lactose absent + glucose present

Answer 15

1) Lactose present + glucose present
- No repressor or activator bound
- Lac Operon = ON (a little)

2) Lactose present + glucose absent
- No repressor bound
- Activator bound
- Lac Operon = ON

3) Lactose absent + glucose present
- Repressor bound
- No activator bound
- Lac operon = OFF

4) Lactose absent + glucose present
- Repressor and activator bound
- Lac operon = OFF

Question 16

Basal Transcription Vs Enhanced Transcription

Answer 16

Basal Transcription:
-Allows a low level of expression

Enhanced Transcription:
-Enhancer trans factors binding to the enhancer
element mediate the response and regulate which genes are expressed at a given point in time

-Bending of DNA allows enhancer binding
proteins+/- mediator proteins to interact with the
basal transcription machinery

Question 17

Transcription factors which bind the enhancer/response elements

Answer 17

1) DNA Binding Domain - Transcription factors have DNA binding domains that only bind to certain DNA sequences
2) Dimerization Domain - Two transcription factors bind together to form a functional DNA binding unit called a dimer
3) Activation/repression domain - Binds to the enhancer binding proteins or other transcription factors and modulates their function

Question 18

Why is it useful for different genes posses the same Response Element?

Answer 18

1) Provides a mechanism to COORDINATE gene regulation
2) Allows multiple genes to be regulated together
3) Allow different genes to be turned on and off at the same time