Lecture 8 - Catabolite Repression and Transcriptional Termination

Question 1

What happens with the activity of enzymes for lactose metabolism when you add lactose to the medium?

Answer 1

The activity of the enzymes increases

Question 2

What happens to the synthesis of lactose when glucose is added to the medium and why?

Answer 2

The synthesis of lactose goes down when glucose is added because glucose turns off the operon

Question 3

What happens to the activity of enzymes when they are involved in the activity of any sugar?

Answer 3

The activity of enzymes increases when involved in metabolism of any sugar

Question 4

What represses the metabolism of sugar?

Answer 4

The addition of glucose to the medium represses the activity of enzymes involved in the metabolism of any sugar.

Question 5

What is the growth rate of E. coli on various carbon sources (glucose, lactose, arabinose and glycerol)?

Answer 5

• Glucose: growth rate = 30 minutes
• Lactose: growth rate = 50 minutes
• Arabinose: growth rate = 55 minutes
• Glycerol: growth rate = 70 minutes

Question 6

`What are you able to determine by looking at the growth rates of E. coli with various carbon sources (glucose, lactose, arabinose and glycerol) ?

Answer 6

• able to determine how much sugar is derived from each of these sources

Question 7

Is more energy extracted from glucose than lactose?

Answer 7

• No, but yes for the net energy in the cell because the net energy on glucose is much higher than that in lactose

Question 8

What is a constitutively expressed enzyme and what is an example of one?

Answer 8

• They are enzymes that are being made, or are on all the time, an example is glycolytic enzymes

Question 9

What is an example of an enzyme that is induced, or only turned on when needed?

Answer 9

beta-galactosidase

Question 10

What are the effects that cAMP has on the growth rate of E. coli on various carbon sources?

Answer 10

• as growth rate slows, levels of cAMP increases
• cAMP levels are low in glucose
• cAMP levels are high in lactose, arabinose and glycerol
• cAMP binds to the regulatory protein
• the levels of cAMP depends on rates it’s synthesized and rates it’s degraded

Question 11

What inhibits the activity of the enzyme that synthesizes cAMP?

Answer 11

glucose

Question 12

What is the name of the enzyme that synthesizes cAMP?

Answer 12

adenylate cyclase

Question 13

What is required for maximal initiation of the lactose operon and other operons that code for enzymes required for sugar metabolism in E. coli?

Answer 13

the catabolite activator protein (CAP)

Question 14

What is present when the operon is off because the CAP is not bound?

Answer 14

glucose and lactose

Question 15

What is present when the operon if off both becaue lac repressor bound and because CAP not bound?

Answer 15

glucose

Question 16

What is present when the operon is turned off because of just the lac repressor being bound?

Answer 16

nothing (neither glucose or lactose)

Question 17

What is present when the operon is on?

Answer 17

lactose

Question 18

When is the only time you see transcription from lactose operons?

Answer 18

when just lactose is present

Question 19

What causes the RNApolymerase to pause?

Answer 19

a hairpin loop in the RNA

Question 20

What does a rho-independent terminator contain?

Answer 20

an inverted repeat followed by a string of approximately six adenine nucleotides

Question 21

When does transcription terminate?

Answer 21

when inverted repeats form a hairpin followed by a string of uracils

Question 22

How does rho-independent and rho-dependent terminators differ?

Answer 22

in the GC content of the stem structure and the uracils in the RNA hydrogen-bonded to the adenines of the DNA template

Question 23

What are the steps of rho-dependent termination of transcription?

Answer 23

1) rho binds to an unstructured region of RNA and moves towards its 3’ end, following RNA polymerase
2) when RNA polymerase encounters a terminator sequence, it pauses and rho catches up
3) using helicase activity, rho unwinds the DNA-RNA hybrid and brings transcription to an end

Question 24

What does a mutation in the sequence of a rho-independent termination result in?

Answer 24

the dependence of termination of rho at this site

Question 25

Depending on the abundance of rho, different mRNAs are produced generating what?

Answer 25

different polypeptides encoded by a single operon