Global Gene Regulation Flashcards
What is global gene regulation?
Coordinated regulation of transcription of several different genes and operons at the same time, often by the same regulatory protein.
Where can an early example of global gene regulation be seen?
In catabolite repression which refers to an effect called the “glucose effect”.
What is the glucose effect?
It is when the presence of glucose in the growth medium will repress the expression of other sugar operons like the Lac operon.
What happens if you grow E. coli in a broth containing an equal amount of two different sugars, glucose plus lactose?
You will see a “diauxic growth curve”
Why does the glucose effect happen?
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.
The presence of glucose represses the expression of what in E. coli?
4 different sugar (fermentation) operons:
1. The lac operon
2. The galactose operon
3. The maltose operon
4. The arabinose operon
Why can the presence of glucose repress the expression of certain other sugar operons in E. coli?
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).
What is the effector molecule that the function of CAP (catabolite activator protein) is dependent (modulated) upon?
cAMP (cyclic adenosine monophosphate)
In bacteria, cAMP contains what? Additionally, what is cAMP synthesized by?
A 3’-5’ cyclic phosphate group. It is synthesized by the bacterial enzyme “adenylate cyclase”.
What type of regulatory protein is CAP?
It is an activator type transcriptional regulatory protein.
What does CAP recognize and bind to?
An activator binding site (specific sequence code) called the “CAP binding site” (or CAP box).
Where is the CAP binding site (CAP box) found?
Upstream of the promoter in each of the 4 sugar operons.
CAP will only bind to the CAP box sequence under what circumstance?
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.
What happens when CAP-cAMP binds to the CAP box sequence (activator site)?
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.
What causes RNA polymerase to bind much tighter to the promoter and thus activate or turn on the transcription initiation of the operon?
The physical contact between CAP protein with the alpha subunit of RNA polymerase.
What does the ability of CAP to activate the transcription of the 4 sugar operons (lac, galactose, maltose, and arabinose) depend on?
The level of cAMP in the bacterial cell.
What does the level of cAMP in a bacterial cell depend on?
The activity of the enzyme “adenylate cyclase”.
What can change the activity of the enzyme adenylate cyclase, therefore affecting the level of cAMP in a bacterial cell?
Glucose
(The glucose effect)
What is the mechanism of the glucose effect?
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.
How is glucose able to change the activity of the enzyme adenylate cyclase?
Because of the way glucose is transported into the bacterial cell.
How is glucose transported into the bacterial cell?
By what is called a “phosphotransferase system” (PTS) of transport.
What is a phosphotransferase system of transport?
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.
How is lactose transported into the bacterial cell?
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.
What happens when there is no transport of glucose into the bacterial cell?
EIIA is mostly phosphorylated
(contains a phosphate group) and will stimulate the activity of adenylate cyclase.
What happens when there is transport of glucose (glucose is present) in the bacterial cell?
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.
Do you feel confident enough to draw out the glucose effect diagram?
If not, then try harder
If yes, then prove it
