Global Gene Regulation

Question 1

What is global gene regulation?

Answer 1

Coordinated regulation of transcription of several different genes and operons at the same time, often by the same regulatory protein.

Question 2

Where can an early example of global gene regulation be seen?

Answer 2

In catabolite repression which refers to an effect called the “glucose effect”.

Question 3

What is the glucose effect?

Answer 3

It is when the presence of glucose in the growth medium will repress the expression of other sugar operons like the Lac operon.

Question 4

What happens if you grow E. coli in a broth containing an equal amount of two different sugars, glucose plus lactose?

Answer 4

You will see a “diauxic growth curve”

Question 5

Why does the glucose effect happen?

Answer 5

Because glucose is a more efficient source of food for growth. The bacteria will derive a larger net yield of ATP energy per glucose molecule than from lactose and glucose will support a faster growth rate than lactose.

Question 6

The presence of glucose represses the expression of what in E. coli?

Answer 6

4 different sugar (fermentation) operons:
1. The lac operon
2. The galactose operon
3. The maltose operon
4. The arabinose operon

Question 7

Why can the presence of glucose repress the expression of certain other sugar operons in E. coli?

Answer 7

Because all 4 of these sugar operons (lac operon, galactose operon, maltose operon, and arabinose operon) are controlled by the same regulatory protein CAP (catabolite activator protein).

Question 8

What is the effector molecule that the function of CAP (catabolite activator protein) is dependent (modulated) upon?

Answer 8

cAMP (cyclic adenosine monophosphate)

Question 9

In bacteria, cAMP contains what? Additionally, what is cAMP synthesized by?

Answer 9

A 3’-5’ cyclic phosphate group. It is synthesized by the bacterial enzyme “adenylate cyclase”.

Question 10

What type of regulatory protein is CAP?

Answer 10

It is an activator type transcriptional regulatory protein.

Question 11

What does CAP recognize and bind to?

Answer 11

An activator binding site (specific sequence code) called the “CAP binding site” (or CAP box).

Question 12

Where is the CAP binding site (CAP box) found?

Answer 12

Upstream of the promoter in each of the 4 sugar operons.

Question 13

CAP will only bind to the CAP box sequence under what circumstance?

Answer 13

When CAP is bound to the effector molecule cAMP. Thus, CAP-cAMP will bind to the CAP box sequence upstream of the promoter in each operon.

Question 14

What happens when CAP-cAMP binds to the CAP box sequence (activator site)?

Answer 14

CAP-cAMP will bend the DNA allowing the CAP protein to make physical contact with the alpha subunit of RNA polymerase bound to the promoter.

Question 15

What causes RNA polymerase to bind much tighter to the promoter and thus activate or turn on the transcription initiation of the operon?

Answer 15

The physical contact between CAP protein with the alpha subunit of RNA polymerase.

Question 16

What does the ability of CAP to activate the transcription of the 4 sugar operons (lac, galactose, maltose, and arabinose) depend on?

Answer 16

The level of cAMP in the bacterial cell.

Question 17

What does the level of cAMP in a bacterial cell depend on?

Answer 17

The activity of the enzyme “adenylate cyclase”.

Question 18

What can change the activity of the enzyme adenylate cyclase, therefore affecting the level of cAMP in a bacterial cell?

Answer 18

Glucose

(The glucose effect)

Question 19

What is the mechanism of the glucose effect?

Answer 19

When glucose is present in the growth medium it causes a reduction in activity of the enzyme adenylate cyclase. This in turn lowers the levels of cAMP in the cell and thus CAP is not bound to its effector molecule cAMP. CAP, therefore, can not activate the transcription of these other sugar operons.

Question 20

How is glucose able to change the activity of the enzyme adenylate cyclase?

Answer 20

Because of the way glucose is transported into the bacterial cell.

Question 21

How is glucose transported into the bacterial cell?

Answer 21

By what is called a “phosphotransferase system” (PTS) of transport.

Question 22

What is a phosphotransferase system of transport?

Answer 22

A very energy efficient means of transporting a sugar into the bacterial cell and is one of the reasons that glucose is the preferred sugar for growth.

Question 23

How is lactose transported into the bacterial cell?

Answer 23

Via a “permease” that uses a “symport” mechanism of transport, which is a much more energy demanding transport system compared to that of glucose with the (PTS) transport system.

Question 24

What happens when there is no transport of glucose into the bacterial cell?

Answer 24

EIIA is mostly phosphorylated
(contains a phosphate group) and will stimulate the activity of adenylate cyclase.

Question 25

What happens when there is transport of glucose (glucose is present) in the bacterial cell?

Answer 25

The protein EIIA is donating its phosphate group to glucose. Thus, EIIA is mostly in the unphosphorylated state. Here, there is no stimulation of adenylate cyclase and the levels of cAMP go down. With low levels of cAMP, CAP is not bound to its effector molecule and will not activate the transcription of the lac operon.

Question 27

Do you feel confident enough to draw out the glucose effect diagram?

Answer 27

If not, then try harder
If yes, then prove it