Part 3: Bacterial Regulation of transcription

Question 1

The preferred carbon source for E. coli is:

Answer 1

• Glucose
• However, cells can utilize alternative carbon sources, including lactose.

Question 2

Lactose is normally present at fewer than 10 molecules per cell, but can be induced more than 1,000-fold in what conditions?

Answer 2

• absence glucose
• presence lactose

Question 3

Lactose is hydrolyzed by:

Answer 3

β-galactosidase

Question 4

β-galactosidase:

Answer 4

• encoded within the lac operon, a cluster of 3 genes that are transcribed as a single mRNA molecule
• hydrolyzes lactose

Question 5

Operon:

Answer 5

• a cluster of functionally-related genes under the control of a single promoter
• transcribed as a single mRNA molecule

Question 6

The two control sites of the lac operon:

Answer 6

• promoter (p): binding site for RNA polymerase
• operator (o): binding site for lac repressor

Question 7

In the absence of lactose, transcription of the lac operon is repressed by:

Answer 7

• product of the lacI gene, the lac repressor.
• Lac repressor binds the operator and sterically blocks RNA polymerase binding to the promoter.

Question 8

How does the lac repressor function?

Answer 8

• binds the operator and sterically blocks RNA polymerase binding to the promoter region of the lac operon.

Question 9

Allolactose:

Answer 9

• a spontaneous lactose metabolite
• binds the lac repressor and causes it to dissociate from the operator, thereby allowing transcription to occur

Question 10

The two proteins that regulated lac operon expression:

Answer 10

1. lac repressor
2. catabolite activator protein (CAP)

Question 11

Catabolite repression is defined as:

Answer 11

• glucose-mediated inhibition of genes encoding catabolic enzymes
  
  • in the presence of the preferred carbon source glucose, genes encoding enzymes involved in metabolism of less desirable carbon sources (like lactose) are repressed.

Question 12

Catabolite repression is relieved by:

Answer 12

cAMP

a “hunger signal”

Question 13

What does the CAP-cAMP complex do?

Answer 13

• binds specific promoter sites resulting in stabilization of the RNA polymerase promoter binding, leading to gene expression.

Question 14

The two molecules needed for the lac operon to be transcribed:

Answer 14

1. allolactose
   
   • a lactose metabolite
   • only present in presence of lactose
   • binds and inhibits lac repressor
2. cAMP
   
   • a “hunger” signal
   • only present in absence of glucose
   • binds to CAP, forms CAP-cAMP complex, which binds to the lac promoter and stabilize RNA polymerase-promoter binding

Question 15

In its most basic form, the CAP-cAMP complex is a:

Answer 15

• transcriptional activator
  
  • stabilizes RNA polymerase promoter binding

Question 16

lac operon expression only occurs in what conditions?

Answer 16

1. ABSENCE glucose
2. PRESENCE lactose

Question 17

Operators usually overlap:

Answer 17

• the promoter region, thereby blocking RNA polymerase from binding to the promoter in the presence of a repressor bound to the operator.

Question 18

Schematic of a general bacterial operon:

Answer 18

Question 19

What binds RNA polymerase to the promoter region of the lac operon?

Answer 19

• sigma subunit of the holo-enzyme
• absence of the lac repressor (allolactose)
• presence of CAP-cAMP complex (cAMP)

Question 20

Common theme in transcription factor structural motifs:

Answer 20

• proteins bind palindromic sequences as dimers