Lecture 9 - Transcriptional Regulation 1

Question 1

Ways in which gene expression is regulated in prokaryotes

Answer 1

1. Downregulate transcription
2. Hydrolyze mRNA, preventing translating
3. Prevent mRNA translation at the ribosome
   4 Degrade the protein after it is made
4. Inhibit the function of the protein

Question 2

Selective gene transcription

Answer 2

Not all genes are transcribed any given time. A ‘decision’ is made about which genes to activate

Question 3

Two types of regulatory proteins

Answer 3

1. Repressor proteins

2. Activator proteins

Question 4

Negative regulation

Answer 4

In negative regulation, the gene is normally transcribed. Binding of a repressor protein prevents transcription.

Question 5

Positive regulation

Answer 5

In positive regulation, the gene is normally not transcribed. An activator protein binds to stimulate transcription.

Question 6

Inducers

Answer 6

Compounds that stimulate the synthesis of a protein

Question 7

Inducible proeins

Answer 7

Proteins that are produced once inducers work

Question 8

Constitutive proteins

Answer 8

Proteins that are made all the time t a constant rate

Question 9

Allosteric regulation

Answer 9

Feedback from the end product of a metabolic pathway can block enzyme activity

Question 10

Transcriptional regulation

Answer 10

Feedback from the end product of a metabolic pathway can stop the transcription of genes that code for the enzymes in the pathway

Question 11

Structural genes

Answer 11

Gene that encode the primary structure of a protein not involved in the regulation of gene expression. Can be transcribed into mRNA.

Question 12

Operon

Answer 12

A cluster of genes with a single promoter that are transcribed together into a single mRNA

Question 13

What does a typical operon consist of?

Answer 13

1. Promoter
2. Operator
3. 2 or more structural genes

Question 14

Operator

Answer 14

Short stretch of DNA that lies between the promoter and the structural genes. Can bind very tightly with regulatory proteins that either activate or repress transcription

Question 15

Three ways to control the transcription of operons

Answer 15

1. An inducible operon regulated by a repressor protein (deault is ‘off’)
2. A repressible operon regulated by a repressor protein (default is ‘on’)
3. An operon regulated by an activator protein (default is ‘off’)

Question 16

Inducible systems

Answer 16

The substrate of a metabolic pathway (the inducer) interacts with a regulatory protein (the repressor), rendering the repressor incapable of binding to the operator and thus allowing transcription.
Turned on only when the substrate is available

Question 17

Repressible systems

Answer 17

The product of a metabolic pathway (co-repressor) binds to a regulatory protein, which is then able to bind to the operator and block transcription.
Turned on until the concentration of the product becomes excessive

Question 18

Catabolite repression

Answer 18

A system of gene regulation in which the presence of the preferred energy source repressed other catabolic pathways

Question 19

Activator protein

Answer 19

Can stimulate the frequency of initiation of transcription by RNA polymerase

Question 20

Three proteins involved in uptake and metabolism of lactose (a B-galactoside)

Answer 20

B-galactoside permease: a carrier protein that moves sugar into the cell.
B-galactosidase: an enzyme that hydrolyses lactose (to glucose and galactose).
B-galactoside transacetylase: transfers acetyl groups to certain B-galactosides.

Question 21

When does E. coli synthesize the three lactose enzymes?

Answer 21

If lactose is predominant and glucose is low it synthesizes all three enzymes

Question 22

What do Operons allow for?

Answer 22

Operons allow for genes to be co-regulated. Each operon usually encodes genes involved in the same overall function/pathway

Question 23

Explain the lac operon when Lactose is absent

Answer 23

1. The repressor protein encoded by the regulatory gene prevents transcription by binding to the operator
2. RNA polymerase cannot bind to the promoter; transcription is blocked
3. No mRNA is produced, so no enzyme is produced

Question 24

Explain the lac operon when Lactose is present

Answer 24

1. Lactose induces transcription by binding to the repressor, which then cannot bind to the operator.
2. RNA polymerase binds to the promoter
3. RNA polymerase can then transcribe the genes for enzymes

Question 25

What type of mRNA is made from the lac operon?

Answer 25

Polycistronic mRNA. Meaning this mRNA codes for more than one polypeptide separately within the same RNA molecule

Question 26

Example of an inducible operon

Answer 26

lac operon

Question 27

Example of a repressible operon

Answer 27

trp operon

Question 28

Example of an operon regulated by an activator protein

Answer 28

If high lactose but low glucose, CRP (cAMP receptor protein) binding to the lac operon promoter makes the sigma factor-RNA polymerase promoter binding more efficient, and increases (‘activates’) transcription

Question 29

Example of catabolite repression

Answer 29

If there are high levels of glucose and some lactose is present
1. When glucose levels are high, cAMP is low and CROP doesn’t bind the promoter. RNA polymerase cannot bind effectively
2 Transcription of the structural genes for lactose-metabolizing enzymes is reduced