Lecture 8: Regulation of Prokaryotic Gene Expression 1

Question 1

Transcription leads to

Answer 1

-mRNA degradation
or
-translation –> protein processing & degradation

Question 2

Prokaryotic genes are often organised in

Answer 2

functional operons

Question 3

prokaryotic cell function operon description:

Answer 3

Promoter region
-5' end 
-RNA polymerase attaches
-operator 
Functional genes
-Gene A,B&C 
-3' end

Question 4

Prokaryotes:

Answer 4

bacteria / archaea

Question 5

Transcription direction:

Answer 5

5’ —> 3’

Question 6

Gene operons are often controlled by

Answer 6

activators or repressors

Question 7

Activators and gene operon:

Answer 7

POSITIVE REGULATION

• Activator protein binds to promoter region on operon. Stimulates movement of RNA polymerase. Genes are transcribed.
• Activator kicks back of RNA polymerase. Choppy to the left

Question 8

Activators and gene operon: if activator gene is not in sufficient quantity or is faulty

Answer 8

• if faulty is degraded

- RNA polymerase is not stimulated & transcriptional induction of operon is prevented

Question 9

Repressors and gene operon:

Answer 9

• NEGATIVE REGULATION
• If the repressor protein is not in sufficient quantity then the operon is expressed.
• as promoter binding site is free & RNA polymerase pathway is not blocked.
• choppy to the right

Question 10

Repressors and gene operon: in sufficient quantity

Answer 10

If the repressor gene is in sufficient quantity then it binds to the binding site on promoter and block the route of polymerase RNA

Question 11

Lac Operon what type is it

Answer 11

negative regulation; using repressor.

Question 12

Lac operon as example:

NO LACTOSE PRESENT IN GROWTH MEDIUM

Answer 12

-repressor binds to promoter binding site and RNA polymerase pathway is blocked - transcription does not occur

Question 13

Lac operon as example: LACTOSE PRESENT IN GROWTH MEDIUM

Answer 13

Lactose binds to repressor and this causes it to change shape, meaning it can no longer bind to site on operator region. RNA polymerase pathway is clear so genes are transcribed

Question 14

Lac operon 3 genes:
Lac Z
Lac Y
Lac A

Answer 14

Lac Z = Beta-galactosidase
Lac Y = permeate
Lac A = transacetylase

Question 15

what does beta-galactosidase do?

Answer 15

• mediates hydrolysis of lactose
• Lactose –LacZ–> H2O + D-galactose + D-Glucose
• ..so induced production of LacZ reduces lactose concentration which represses induction of the Lac operon

Question 16

The Lac operon is under

Answer 16

negative feedback regulation

Question 17

Lac operons negative feedback loop:

Answer 17

• -> Lactose inhibits LacI repressor
• -> Lac operon is expressed
• -> b-galactosidase activity reduces lactose levels
• -> Lac I repressor becomes active
• -> Lac operon expression is blocked
• -> Lactose levels increase
• -> –> –>

Question 18

Trp operon in E.coli uses a

Answer 18

repressor

- negative regulation

Question 19

example Trp operon:

Answer 19

• Repressor gene trp codes for an inactive repressor protein trpR
• meaning RNA polymerase pathway is not blocked transcription occurs.
• Genes transcribed produce tryptophan synthesis protein complex –> tryptophan
• tryptophan binds to inactive repressor protein trpR, changes shape of repressor molecule
• repressor binds to promoter region and blocks RNA polymerase pathway. –> no transcription

Question 20

trp operon is under

Answer 20

Negative feedback regulation

Question 21

negative feedback loop for trp

Answer 21

1) the trpR repressor is inactive
2) the trp operon is expressed
3) Tryptophan biosynthesis increases
4) Tryptophan binds to trpR repressor
5) Trp operon expression is blocked
6) Tryptophan biosynthesis decreases
AND REPEAT.. the trpR repressor is inactive…etc

Question 22

Example: Quorum sensing in bacteria

- at low cell density:

Answer 22

1 bacterial cell, few auto-inducer compound (often homoserine lactone).

-causes individual behaviours

Question 23

Example: Quorum sensing in bacteria

High cell density:

Answer 23

Many bacterial cells & auto-inducer compound (often homoserine lactone)

Causes group behaviours such as:

• Virulence
• Microfilm formation
• Luminescence

Question 24

Quorum sensing occurs in:

Answer 24

Aliivibrio fischeri

Question 25

Aliivibrio fischeri produce what at high cell density but not at low cell density

Answer 25

LIGHT

Question 26

Bioluminescence is controlled by the

Answer 26

LuxCDABEG operon

Question 27

example animal for quorum sensing:

Answer 27

Euprymna Scolopes (the Hawaiian bobtail squid) due to Alivibrio Fischeri

Question 28

Example quorum sensing at low A. fischeri density

Answer 28

• LuxR & LuxI are constantly transcribed
• Homoserine lactone is secreted
• Homoserine lactone levels do not build up
• No binding of LuxR to Lux box (operon)
• No transcription of lux operon

Question 29

Example quorum sensing at high A. fischeri density

Answer 29

• LuxR & LuxI are constantly transcribed
• Homoserine lactone is not sufficiently secreted
• Homoserine lactone levels build up in cell, some from the environment
• Binding of LuxR to lux box as homoserine has corrected shape of LuxR so they are now compatible
• Transcription of Lux operon
• LUMINESCENCE