L18/19: Gene Regulation - lac operon

Question 1

What are the 3 genes the lac operon contains?

Answer 1

• lacZ
• lacY
• lacA

Question 2

What are the 3 resulting proteins in the lac operon and their functions?

Answer 2

LacZ protein:

• beta-galactosidase
• cleaves lactose into glucose and galactose

LacY protein:

• transporter
• transporter lactose into the cell

LacA protein:

• trans-acetylase
• not clear what its role in the lac operon is and is found NOT to be necessary for lactose metabolism
• often not shown on the operon!

Question 3

What gene can be found upstream of the lac operon and which protein does it code for?

Answer 3

• lacI

- produces the regulatory protein, LacI

Question 4

Is LacI a positive or negative regulatory protein?

Answer 4

• Negative

Question 5

Is the promoter for the lac operon strong or weak?

Answer 5

• Strong

Question 6

The promoter binds sigma/RNA polymerase… (strongly or weakly)

Answer 6

• Strongly

Question 7

What is the signal molecule?

Answer 7

• lactose (inducer)

Question 8

Where is the operator region found?

Answer 8

• Downstream but overlapping the promoter

Question 9

Is LacI constitutively expressed or is it environmentally-regulated?

Answer 9

• Constitutively expressed

Question 10

Describe the LacI situation without lactose.

Answer 10

• the regular form of LacI can bind to the operator, which prevents/blocks RNA pol from binding
• this results in no transcription - it’s a repressor!

Question 11

Describe the LacI situation in the presence of lactose.

Answer 11

• this form of LacI cannot bind to the operator

- no repression, RNA pol can bind and transcription of lacZYA operon

Question 12

Where does the lactose bind?

Answer 12

• binds to LacI proteins which then results in promoting transcription

Question 13

In negative regulation and in the absence of the signal molecule, what is the general role of LacI?

Answer 13

• bind to an operator downstream of the promoter and prevents transcription