Regulation of gene expression

Question 1

Regulation of gene expression

5 points

Answer 1

1. Temporal variation in gene expression / protein synthesis
2. Organisms go through development stages produced by different sets of proteins, controlled by regulated gene expression
3. allows organisms to “respond” to changes in their environment
   E.g. plant drought…
4. Different genes are expressed depending on environmental or developmental cues
5. All genes are present on the genome, but not all genes expressed all the time

Question 2

Regulation of gene expression can be at multiple points:

Answer 2

1. Before transcription: block/allow initiation of transcription
2. During transcription: less mRNA produced
3. After transcription but before translation: initiation of translation
4. During translation: breakdown the protein after it is produced
5. After translation: inhibit the function of the protein

Question 3

Regulation of gene expression in _____ is generally via the most efficient mechanism:

Answer 3

prokaryotes

Question 4

Regulation of gene expression in prokaryotes

Before transcription: initiation of transcription

Answer 4

1. gene expression starts at Promoter (RNA polymerase binds to initiate transcription)
2. Not all promoters active all the time (selective gene transcription)
3. Activation of gene expression involves 2 types of regulatory proteins that bind to the promoter to regulate gene expression:

Repressor proteins – negative regulation

Activator proteins – positive regulation

Question 5

Regulation of gene expression in prokaryotes

Negative regulation

Answer 5

binding of repressor protein blocks transcription (stops RNA polymerase from binding to promoter).

Question 6

Regulation of gene expression in prokaryotes

Positive regulation,

Answer 6

binding of activator protein stimulates transcription (allows RNA polymerase to bind to promoter).

Question 7

Negative regulation: Bacteria regulate gene transcription to save energy -

E.coli (bacteria in human intestine) adapt to changes in its chemical environment – foodstuff passing through!

3 points

Answer 7

1. Glucose (e.g. from fruit juice) or lactose (from milk)
2. Glucose: preferred energy source, easiest sugar to metabolise
3. Lactose: ß-galactoside – composed of ß-galactose linked to glucose – more complex to metabolise.

Question 8

Negative regulation: Bacteria regulate gene transcription to save energy -

E. coli produces three specific proteins in order to uptake and metabolise lactose:

Answer 8

1. b-galactoside permease – carrier protein, transports lactose into the bacterial cell
2. b-galactosidase – enzyme hydrolyses lactose to glucose + galactose
3. b-galactoside transacetylase – unknown…

These lactose uptake / metabolism proteins are only produced when lactose is present

Question 9

Negative regulation

All genes coding for lactose uptake/metabolism proteins are in a single unit of DNA –

Answer 9

1. lac operon
2. Operon =

unit of DNA contains several genes that are transcribed together,

share the same promoter,

have an operator

Other regulatory sequences – aren’t transcribed, are the site where regulatory protein binds.

Question 10

Repression of lac operon

Gene for the repressor protein (i) is located near the lac operon (repressor protein has its own promoter, Pi)

3 points

Answer 10

1. Promoter – RNA polymerase binds to initiate transcription
2. Operator – repressor protein can bind to repress transcription
3. Functional genes = genes for enzymes involved in lactose metabolism

Question 11

Repression of lac operon

Two possible environments that affect whether 3 “structural genes” are expressed:

Answer 11

1. Lactose absence

Repressor protein (gene i) prevents transcription by binding to the operator

RNA polymerase cannot bind to the promoter; transcription is blocked

2. Lactose presence

allolactose forms from lactose inside the bacterial cell and induces transcription:

Allolactose binds to the repressor:
changes the shape of the repressor – it cannot bind to the operator. RNA polymerase binds to the promoter.

RNA polymerase can then transcribe the genes for the enzymes.

Question 12

Summary

Absence/presence of lactose influences

Answer 12

transcription of lactose metabolising enzymes via negative control - transcription is decreased in presence of repressor protein

Question 13

Summary

E.coli can also use

Answer 13

positive control – transcription is increased in presence of activator protein

Question 14

Summary

Lac operon

Answer 14

relative levels of both lactose and glucose determine the amount of transcription

Question 15

Two environments that affect rate of lac operon transcription

Answer 15

A) low glucose,

B) high glucose

Question 16

Lac operon

In presence of lactose

Answer 16

lac repressor cannot bind, = production of lactose metabolising enzymes

Question 17

Lac operon

Metabolising lactose is only important when

Answer 17

glucose is low / not present

Glucose is preferred energy source, easiest sugar to metabolise

Question 18

positive regulation can increase efficiency of lac operon transcription when

Answer 18

glucose levels are low

Not required when glucose levels are high. positive regulation not turned on

Question 19

Positive regulation of lac operon

Low glucose (lactose present)

3 points

Answer 19

1. A regulatory protein (CRP) binds to cAMP (cyclic Adenosine MonoPhosphate), and the CRP-cAMP complex binds to the promoter
2. RNA polymerase then binds more efficiently to the promoter
3. The lac operon
   structural genes are transcribed

Question 20

Positive regulation of lac operon

High glucose (lactose present)

Answer 20

1. cAMP is low, CRP is inactive and does not bind to the promoter. RNA polymerase does not bind efficiently
2. Transcription of structural genes is reduced

Question 21

Positive regulation of lac operon

see summary table on desktop

Answer 21

.

Question 22

prokaryotic vs eukaryotic gene expression regulation

Locations of functionally related genes

Answer 22

Clustered in operons with 1 promoter

vs.

Distant from each other with separate promoters

Question 23

prokaryotic vs eukaryotic gene expression regulation

RNA polymerases

Answer 23

One

vs.

Three:
I transcribes rRNA
II transcribes mRNA
III transcribes tRNA and small RNAs

Question 24

prokaryotic vs eukaryotic gene expression regulation

Promoters & regulatory sequences

Answer 24

Few

vs.

Very many!

Question 25

prokaryotic vs eukaryotic gene expression regulation

Initiation of transcription

Answer 25

Binding of RNA polymerase to promoter

vs.

Binding of many proteins – transcription factors and RNA polymerase

Question 26

In Eukaryotes, _______ regulate gene expression

Answer 26

transcription factors (TFs)

Question 27

Eukaryotes

different types of TFs that bind to “regulatory elements”:

Answer 27

1. enhancers – activate / increase rate of transcription
2. silencers – bind factors that repress transcription
3. Promoter can be far (few hundred base pairs) upstream from transcription start site. Regulatory elements can be thousands of base pairs away.
4. When TFs bind to these regulatory elements, they interact with RNA polymerase complex, causing DNA to bend

Question 28

Eukaryotes (transcription factors)

e.g. Action of enhancers

3 points

Answer 28

1. specific TFs bind to enhancer and regulatory protein sites
2. DNA bending allows specific TFs to interact with RNA polymerase complex and affect the rate of transcription
3. Note: Regulatory elements that bind specific TFs can be thousands of base pairs away from transcription start site

Question 29

TFs can co-ordinately regulate the expression of sets of genes:

2 points

Answer 29

1. Multiple genes may need to be expressed at the same time for a biochemical pathway to function.
2. Prokaryotes – related genes contained in an operon that share the same promoter (e.g. lac operon)
3. Eukaryotes – no operon, genes far away with their own promoters…

Expression of related genes can be coordinated if they have regulatory sequences that bind the same TF

Mode of regulation for coordinating production of stress response proteins – e.g. Plant response to drought

Question 30

Plants response to drought

3 points

Answer 30

1. Under drought stress, plants must simultaneously produce multiple proteins whose genes are scattered throughout the genome (across different chromosomes).
2. The synthesis of these proteins is called “stress response” – only reproduced when plant is under drought stress
3. To coordinate expression, each stress gene has a specific
   regulatory sequence near its
   promoter called the stress response element (SRE)

Question 31

steps for plants response to drought

Answer 31

1. A stressor (drought) activates transcription of a gene that encodes a transcription factor
2. Transcription factor binds to stress response element (SRE) of each gene, which stimulates transcription of each gene that has the same SRE sequence (genes A, B, C…)
3. These genes produce different proteins that have a role in the stress response

In drought stress these stress response proteins:

Help plant to conserve water
Are important in agriculture – crops are often affected by adverse weather