2) Control of Transcription Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

Why is it important that the amount a gene is transcribed and translated be varied?

A

Not all proteins are required in the same amounts or at the same time

Switching transcription of genes on and off when needed SAVES ENERGY

E.coli: Only needs to transcribe genes needed to make proteins to use the food substrates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How do prokaryotes control the amount of transcription?

A

1) Cell can control which genes are transcribed (where in the DNA transcription starts and stops)
2) Can control the rate and amount of transcription

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are the 3 different types of control mechanisms used by bacteria? (Names Only)

A

1) Operons: Coordinate control of gene groups
2) Alternative sigma factors: Decides which genes are transcribed
3) Regulating transcription termination: Decides whether additional genes are transcribed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are operons?

What are the 2 types?

A

Genes which have related function in bacteria are grouped together and are regulated in a coordinate fashion= Operon

Controlled by a shared promoter

Regulatory proteins bind to the DNA of the promoter and its operator to regulate expression

Example: 9 genes making the 9 enzymes which synthesise histidine= Operon

Can be controlled by positive or negative mechanisms

2 types:

1) Catabolic (e.g. lac operon)
2) Biosynthetic (e.g. Trp operon)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the difference between positive or negative control?

A

The DNA-binding protein that binds the promoter can be a positive or negative regulator

Positive= Transcription is switched ON

Negative= Transcription is switched OFF

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What the difference between catabolic and biosynthetic operons?

A

Catabolic: When the operon is involved in breaking down substrates. Produce molecules they can use for extracting energy

Biosynthetic: Synthesising new molecules they require for life

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Where are catabolic operons used?

A

Control expression of enzymes used in sugar metabolism and utilisation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What happens to catabolic operons when glucose is available?

A

Glucose= Preferred sugar
Do not needed to switch on expression of enzymes which break down other compounds and release sugars= Transcription= OFF, normally OFF

These operons (such as Lactose (lac) and arabinose (are) are ONLY switched on when their sugar is present

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What does the lactose operon do?

A

Lac gene: Codes for the enzyme B-galactosidase which breaks down the sugar lactose= Cleaves the beta 1,4 linkage

Lactose: Disaccharide made up of 2 monosaccharide sugars joined together= Glucose + Galactose= E.coli can only use the monosaccharides

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the different genes of the lac operon?

A

lacZ, lacY and lacA= Lac operon AND also lacI

Structural genes and are transcribed from a single promoter into polycistronic mRNA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the difference between structural and regulatory genes?

A

Structural genes: Code of enzymes or others that actually DO the work

Regulatory genes: Control the operon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the regulatory gene for the lac operon?

A

LacI= Gene that produces the regulatory protein. It has its own promoter and terminator

Product of lacI regulates expression of lacZYA

If its function is disrupted, all of the other 3 genes are expressed ALL the time

It encodes a REPRESSOR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What does lacZ do?

A

Encodes the enzyme beta-galactosidase which is very stable

Proof experiments: Always monitor lacZ as it is the easiest to detect= Only need to monitor one of them
Able to measure beta galactosidase activity easily= turns blue on X-gal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What does lacY do?

A

Encodes a permease= Transports lactose into the bacteria as it cannot diffuse through the plasma membrane by itself

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What does lacA do?

A

Encodes an enzyme transacetylase= Add acetyl groups to toxic sugars= Makes them harmless
(Although no one really knows its function…)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

How is the lac operon regulated when lactose is ABSENCE?

A

Also have: lacO= Next to lacZ, is the operator sequence= Site at which the repressor protein acts

1) lacI gene is transcribed= produces mRNA that gets translated to a protein
2) 4 copies of proteins aggregate together= Tetramer
3) IF THERE IS NO LACTOSE: tetramer can bind to the operator region (lacO)
4) Binding= Interferes with binding of RNA polymerase to the promoter by steric hinderance (gets in the way…)
5) Prevents the expression of operon= Transcription is switched OFF

Binding sites of RNA polymerase at the promotor and lac repressor OVERLAP= if protein binds= Gets in the way= RNA polymerase cannot bind

17
Q

How is the lac operon regulated when lactose is PRESENT?

A

Prevents the repression mechanism from working

1) Always have some permease in the plasma membrane= Imports lactose
2) Beta-galactosidase can rearrange lactose= Produces side product allolactose= Inducer
3) Allolactose binds to lac repressor protein= Conformational change= inactive form that cannot bind to operator
4) RNA polymerase can bind to the promoter and transcribes the operon= Structural genes are transcribed and translated into enzymes

Allolactose is present whenever lactose is present

18
Q

What does the lacI- mutant do?

A

lacI- mutant version of lacI gene= Produces version of the lac repressor protein which CANNOT BIND to the operator

Result: Expression of the 3 structural genes of lac operon are ALWAYS switched on= Transcription ON

lacZ gene is constitutively expressed= Their effect is to cause constitutive expression of lacZ and the other genes

19
Q

How did Jacob and Monod discover the Operon model?

A

Took 2 strains of chromosomes:
F’= Extra inserted genes delivered on a small plasmid
+= Normal, effective wild type version
-= Defective mutant
1) lacI- lacZ+ / F’ lacI lacZ-
Chromosome has mutant lacI, F’ plasmid adds normal lacI joined to mutant lacZ

2) Lac+ lacZ+/ F’lac- lacZ-
Chromosome has normal versions of both genes, F’ plasmid adds mutant version os both

Asked question: Would extra copies of genes (on F’ plasmid) alter behaviour of original strain

20
Q

What happened in strain 1?

A

Normal lacI gene on F’ plasmid makes normal version of repressor protein= Normal repression and induction
SHOWS: lac+ (wild type) is the DOMINANT as it can overcome the lac- mutation + the gene must encode a diffusible product that can move to the operator on the bacterial chromosome= MUST have the lac repressor protein

Strain 2: Also normal induction

21
Q

What does it mean by “lacI acts in TRANS with respect to the lac operon’?

A

The lacI gene does not need to be on the same piece of DNA as the lac operon as there is a diffusible product that can move to the DNA when required= Lactose repressor

22
Q

What do lacO^c mutations do?

A

Cis-dominant

LacO^c = cannot be recognised by a functional wild type repressor (lacI+)= Protein cannot bind= Operon is constitutively switched on

23
Q

How did their cis-dominant property be discovered by Jacob and Monod?

A

Again took 2 strains:
1) lacOc lacZ+ / F’lacO+ lacZ-
Mutant and normal wild type lacZ is on bacterial chromosome, F’ plasmid adds normal wild type lacO operator with mutant lacZ

RESULT: Still constitutive, adding the extra wild type lacO does not help

(ii) lacO+ lacZ+ / F’lacOc lacZ-
Normal wild types on bacterial chromosome, and mutants on F’ plasmid added
RESULT: Normal, the 2 mutations on the plasmid do not destroy normal functioning

SHOWED: When lacO gene is on the SAME chromosome as the functional lacZ, it is dominant= Cis-dominant
LacO cannot produce a product that can diffuse and affect genes somewhere else

24
Q

What does the lacI^s mutations do?

A

Super-repressed mutations and are un-inducible

RESULT: Make repressor insensitive to inducer= Mutant repressor will always be bound to the operator= Transcription is always switched OFF