Wright Lecture 7: Control of Prokaryotic Operons

Question 1

Induction of lactose operon by lactose

Answer 1

1. Lack of beta-galactoside in cell causes lactose build up as allolactose (isomer)
2. Allolactose binds to lactose repressor protein at allolactose binding domain
3. Lactose repressor protein undergoes conformational change
4. DNA-binding domain is masked and transcription can occur

Question 2

Jacob and Monod determination of lactose operon mechanism

Answer 2

Use mutants in lactose operon and its regulatory gene LacI

Construction of partial diploids (merodiploids) with mutations in lactose operon and regulatory gene LacI

Question 3

lacI^o

Answer 3

No repressor

Beta-galactosidase activity with or without lactose

Question 4

lacI^s

Answer 4

Super repressor

No beta-galactosidase activity with or without lactose

Question 5

lacl-

Answer 5

Mutant repressor proteins that cannot bind to opeteraor

Beta-galactosidase activity with or without lactose: constitutively “on”

Question 6

lacO^c

Answer 6

Defective operator: repressor protein cannot bind

Beta-galactosidase activity with or without lactose: constitutive synthesis

Question 7

lacZ-

Answer 7

No enzyme

No beta-galactosidase activity with or without lactose

Question 8

lacP-

Answer 8

Defective promoter

No beta-galactosidase activity with or without lactose

Question 9

Partial diploid

Answer 9

Bacteria chromosome with gene, and plasmid containing another copy of same gene

Question 10

Merodiploid

Answer 10

Partial diploid

Question 11

I+P+O^cZ+/I^sP+O+Z+

Answer 11

Phenotype: constitutive

Question 12

I+P+O^cZ-/I^sP+O+Z+

Answer 12

Phenotype: non-inducible

Question 13

I+P+O^cP+/I^sP-O+Z+

Answer 13

Phenotype: constitutive

Question 14

I+P+O+Z+/I-P-O+Z+

Answer 14

Phenotype: inducible

Question 15

Inducible operon

Answer 15

Lactose causes induction of synthesis of enzyme: more enzymes are needed for metabolic process
Catabolic
Substrate is effector molecule

Question 16

Repressible operon

Answer 16

Tryptophan added causes repression of synthesis of enzyme: less enzyme is needed for metabolic process
Anabolic
End-product is effector molecule for metabolic pathway

Question 17

Repression of tryptophan operon

Answer 17

Repression of tryptophan operon by amino acid tryptophan
Binds to repressor protein, which causes conformational change of
protein to bind DNA and repress transcription

Question 18

Negative control

Answer 18

Prevention of transcription

Question 19

Positive control

Answer 19

Instigation of transcription

Question 20

Arabinose operon

Answer 20

Inducible operon under positive control

Arabinose binds to activator which binds to operator and instigates transcription

Question 21

Repressible operon under positive control

Answer 21

Active activator bound to DNA

Turns off transcription when the activator is bound to ligand

Question 22

Inducible operon under positive control

Answer 22

Transcription not occurring

Ligand binds to inactive activator: activated and binds to DNA to instigate transcription

Question 23

Inducible vs. Repressible operons

Answer 23

Ligand (substrate from catabolic, end-product from anabolic) either induces or represses synthesis of enzymes encoded by the operon

Question 24

Negative vs. Positive control

Answer 24

Regulatory protein stops or enhances transcriptional initiation of the operon

Question 25

3 rules of prokaryotic operon regulation

Answer 25

1. Cells do not waste energy: proteins only made when needed
2. Inducible vs. repressible
3. Negative vs. positive

Question 26

Four mechanisms for prokaryotic control of operons

Answer 26

1. Negative inducible (lactose)
2. Negative repressible (tryptophan)
3. Positive inducible (arabinose)
4. Positive repressible (few examples)