Regulation of Gene Expression - mechanisms

Question 1

Which genes does the lac operon contain?

Answer 1

3 genes that code for proteins involved in digesting lactose:
lacZ, lacY and lacA.

Question 2

Describe the operator region of lac operon

Answer 2

Also known as lacO, this is where a repressor called lacI binds, blocking RNA polymerase from moving on

Question 3

What is the function of the CAP site in the lac operon?

Answer 3

where a cAMP/CAP complex binds to allow RNA polymerase to bind as well.

Question 4

What is the lac operon inducer?

Answer 4

Allolactose, an isomer of lactose

Question 5

Describe the function of allolactose

Answer 5

It binds to the repressor lacI, causing a conformational change that prevents the repressor from binding to the DNA at the operator region.

= means in the presence of lactose RNA polymerase can bind and the lac operon can be transcribed, lactase can then be digested.

Question 6

What is the repressor in the lac operon?

Answer 6

lacl

Question 7

describe the function of LacZ

Answer 7

LacZ is a b-galactosidase, it hydrolyses lactose (the galactose is phosphorylated into glucose phosphate).

Question 8

Describe how the structure of the repressor lacI influences its function

Answer 8

• It’s a homotetramer (4 protein complex)
• each protein has 3 domains, head is used to bind to DNA (the operator), another can bind to allolactose, but both cannot bind at the same time.
• The repressor causes DNA to twist so RNA polymerase cannot actually work (it can bind but that’s it).
• However the RNAP is primed - ready to go - as soon as the inducer binds (so as soon as there’s lactose that needs to be digested.)

Question 9

Describe why the diauxic shift occurs when E.coli are in a medium containing both glucose and lactose.

Answer 9

• E. coli prefer glucose so digest this before lactose = shift
• When glucose present, genes required for digestion of other sugars = turned off
• Glucose inhibits adenylate cyclase (makes cAMP), so when glucose is available there’s no cAMP to bind with CAP, so RNA polymerase will struggle to bind to and transcribe the lac operon

Question 10

In the lac operon, what is the CAP site?

Answer 10

standing for catabolite activator protein - is where a CAP/cAMP complex binds to help RNA polymerase bind and do its thing

Question 11

Lac operon is an example of what?

Answer 11

Transcriptional regulation (in prokaryotes)

Question 12

What is the function of eIF2

Answer 12

Brings charged met tRNA to SSU of the ribosome, once there, GTP is hydrolysed to GDP = conformational change that releases the eIF2 and the GDP from the tRNA

Question 13

What is the function of GAP?

Answer 13

acts as a molecular switch:
a GAP, a GTPase activating protein, is needed to help eIF2 hydrolyse its GTP

Question 14

What is the fucntion of eIF2B?

Answer 14

eIF2B is needed to help eIF2 get rid of that GDP and replace it with another GTP.

Question 15

Why is eIF2 used as a translational control?

Answer 15

This mechanism is used to reduce all protein production in times of cellular stress

Question 16

What happens when you phosphorylate eIF2?

Answer 16

Phosphorylate the eIF2 and its no longer a substrate of the much less abundant eIF2B, it’s an inhibitor, and it binds extra tight, stopping the function of eIF2B (exchange factor to take GDP from elF2)

= transcription stopped