Gene regulation in Bacteria

Question 1

Constitutive Genes

Answer 1

• Expressed all the time, NOT subject to regulation

- Often includes “Housekeeping genes”

Question 2

Repressible Genes

Answer 2

Require a protein to “turn off” expression

Question 3

Inducible Genes

Answer 3

Require a protein to “turn on” expression

Question 4

Transcription Repressor

Answer 4

-protein that turns off expression to repress genes

bind tightly to operators to block RNA polimerase access to the promoter

Question 5

Transcription Activator

Answer 5

• Turns on expression in a gene
• bind to DNA next to promoter
• help RNA polymerase bind to promoter
• When it is time to stop expressing the gene, the activator “falls off” the DNA and RNA polymerase can no longer attach to the promoter for the gene

Question 6

Operator

Answer 6

stretch of DNA that a repressor can bind tightly to so that a gene is repressed

Question 7

How do activators and repressors know where to bind?

Answer 7

“Edges” of base pairs are exposed in DNA

Allows proteins to bind to DNA in a SEQUENCE_ SPECIFIC manner

Question 8

trp Operon

Answer 8

• 5 genes allow bacteria to synthesize tryptophan
• all clustered together in an OPERON
• transcribed together
• 5 enzymes build tryptophan from simple precursors

Question 9

When Tryptophan is present in an E. Coli cell

Answer 9

• no need to express Trp operon
• repressor protein is expressed
• repressor protein binds free tryptophan in the cell
• repressor is only active when it has already bound to tryptophan
• repressor + tryprophan bind DNA at an operator site in the promoter for the trp operon
• *shuts OFF expression in the presence of tryptophan

Question 10

When tryptophan is absent in E. Coli

Answer 10

• the cell needs to express the trp operon
• repressor protein is still expressed but is INACTIVE
• the repressor cannot bind to the DNA in the ansence of trypophan
• the gene is free of repression and RNA polymerase binds to Trp promoter

Question 11

What happens when Glucose is absent in E. Coli cells?

Answer 11

The bacteria starts metabolizing other carbohydrates

Question 12

Lactose:

Answer 12

• Disaccharide
• needs to be hydrolyzed in monosaccharides
• enzyme Beta-galactosidase

Question 13

Three genes of lac operon

Answer 13

1: lacZ- Beta-galactosidase gene that hydrolyzes lactose
2: lacY- lactose permease gene that imports lactose into the cell
3: lacA- Acetylates lactose gene possible role in retaining imported lactose

Question 14

Problem: No Lactose

Answer 14

it doesn’t make sense for E. Coli to express the genes in the lac operon if there is no lactose in the environment of the cell

Question 15

Solution: lacI

Answer 15

• repressor protein in front of the promoter for the Lac operon
• when LacI binds to lactose, LacI dissociates from its operator
• Now the promoter is free of repression: RNA polymerase can bind and transcribe the Lac operon genes, allowing them to be expressed only when lactose is present

Question 16

Problem: plenty of glucose

Answer 16

• it doesn’t make sense for E. Coli to express the genes in the Lac operon if there is plenty of glucose in the environment of the cell
• glucose is easier to break down and use than lactose, so E. Coli would prefer to use glucose instead of lactose if both are present

Question 17

Solution: CAP (catabolite activator protein)

Answer 17

-CAP only binds DNA and works as an activator when the E. Coli cell is starved for glucose
-This is because E. Coli cells starved for glucose make the signaling molecule cAMP
-cAMP binds directly to CAP and they bind to the promoters
no glucose -> cAMP production -> active CAP

Question 18

Lactose absent, glucose present

Answer 18

Lactose present: LacI represses the Lac operon
Glucose present: no cAMP CAP does not bind the Lac operon promoter
Result: No RNA Polymerase Binding -> NO Lac OPeron expression

Question 19

Lactose Absent, Glucose Absent

Answer 19

LacI represses the Lac operon: REPRESSION WINS
Glucose Absent: cAMP -> CAP binds the Lac operon promoter
Both the activator and the promoter binds: No RNA polymerase binding-> no Lac operon expression

Question 20

Lactose present, glucose present

Answer 20

Lactose present: LacI no longer represses the Lac Operon
Glucose present: no cAMP -> CAP does not bind the Lac Operon promotor
nothing binds- Lack of Activation wins
No polymerase binding and no lac operon expression

Question 21

Lactose present, glucose present

Answer 21

LacI does not repress Lac operon, cAMP->CAP binds the Lac operon promotor
Lac Operon Expression