1. Why do genetic classes always bring up the lac operon?
Teachers are genuinely interested in how E. coli metabolizes lactose
B. Teachers want to torture their students with this complicated pathway
C. It was the first example in any organism of how genes were regulated
D. All of the above

2. Lac Operon History
The model of the lac operon represented the first time that gene products (ie proteins!) were shown to regulate the expression of other genes
Developed the right questions: How are genes controlled?
Developed a simple assay to determine if the “Z” gene or “Y” genes of the lac operon were active
Isolated mutations in E. coli to help answer these questions

3. Lac regulatory circuit (lac operon)
The lac genes (3 structural genes total)
Metabolism of lactose requires two enzymes
Permease (gene Y) to transport lactose into the cell
β-galactosidase (gene Z) to modify lactose into allolactose and cleave the lactose molecule to yield glucose and galactose
Transacetylase (gene A) which is not needed for lactose metabolism
Transfers an acetyl group from acetyl-CoA to β-galactosides. Its precise function as part of the lac operon is not understood.
All three are made from one mRNA
Coordinately controlled - genes whose transcription is controlled by a common means
Polycistronic message

4. The Lac Operon: What was known
The gene products from the lac operon were known to be induced in the presence of lactose
Had assays to test for the presence of Beta-galactosidase (Z) and Permease (Y)
They had the ability to make bacteria cells that were haploid (one copy) or diploid (two copies).

5. Lac Operon Foundations
Used a synthetic inducer because allolactose is hydrolyzed by -galactosidase and IPTG is not
IPTG still binds of repressor
Found many mutations that altered gene expression but to study the interactions, they needed diploids
Determine dominance
Created partial diploids by inserting F’ factors

6. What does Cis and Trans mean?
They wanted to know if genes worked in “Cis” or “Trans”
Cis: Operator (O+) works in CIS because it is a DNA sequence that regulates the expression of the lac operon
Trans: Repressor (I) works in TRANS because it is protein product that can travel to regulate the expression of the lac operon

7. DATA: Controls
Lactose Operon is inducible. In the presence of IPTG (a lactose-like chemical), the genes are expressed
O= operator
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

8. DATA: Controls
Lactose Operon is inducible. In the presence of IPTG (a lactose-like chemical), the genes are expressed
O= operator
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

9. DATA: Controls
Lactose Operon is inducible. In the presence of IPTG (a lactose-like chemical), the genes are expressed
O= operator
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

10. Question: Does the operator function in cis or trans?
DATA: Experiment!
Question: Does the operator function in cis or trans?
Lactose Operon is inducible. In the presence of IPTG (a lactose-like chemical), the genes are expressed
O= operator
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

11. Question: Does the operator function in cis or trans?
DATA: Experiment!
Question: Does the operator function in cis or trans?
Lactose Operon is inducible. In the presence of IPTG (a lactose-like chemical), the genes are expressed
O= operator
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

12. How might mutations in O cause a constitutively active phenotype?

13. What does Cis and Trans mean?
They wanted to know if genes worked in “Cis” or “Trans”
Cis: Operator (O+) works in CIS because it is a DNA sequence that regulates the expression of the lac operon
Trans: Repressor (I) works in TRANS because it is protein product that can travel to regulate the expression of the lac operon

14. DATA: Controls Lactose Operon is inducible
In the presence of IPTG (a lactose-like chemical), the genes are expressed
I= Repressor
(I+) is dominant to the I-
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

15. DATA: Controls Lactose Operon is inducible
In the presence of IPTG (a lactose-like chemical), the genes are expressed
I= Repressor
(I+) is dominant to the I-
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

16. DATA: Controls Lactose Operon is inducible
In the presence of IPTG (a lactose-like chemical), the genes are expressed
I= Repressor
I+ is dominant to the I-
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

17. Question: Does the repressor function in cis or trans
DATA: Experiment!
Question: Does the repressor function in cis or trans
Lactose Operon is inducible
In the presence of IPTG (a lactose-like chemical), the genes are expressed
I= Repressor
I+ is dominant to the I-
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

18. Question: Does the repressor function in cis or trans
DATA: Experiment!
Question: Does the repressor function in cis or trans
Lactose Operon is inducible
In the presence of IPTG (a lactose-like chemical), the genes are expressed
I= Repressor
(I+) is dominant to the I-
Z= Lac Z (beta-galactosidase)
Y= Lac Y (permease)

19. How might mutations in I (the repressor) cause a constitutively active phenotype?

20. E. Coli Sigma Factors