1. Gene Regulation
… on / off

2. Looking at Individual Genes
The simplest and most well understood model of the regulation of individual genes is found in prokaryotes.
We will study two examples of gene control in E. coli ; a bacteria found in the human digestive tract

4. Coding Region: Gene(s)
The Operon
Control Region
Coding Region: Gene(s)
Operon

5. The Lac Operon in E. coli Control Region Genes
Respond to the presence or absence of lactose
Produce the enzymes necessary for digesting lactose

6. Each lac gene codes for an enzyme only needed when lactose is present
The Lac Operon in E. coli
Binding site for RNAP
lacZ
lacY
lacA
promoter
operator ON
OFF
Each lac gene codes for an enzyme only needed when lactose is present

7. The Lac Operon in E. coli
Note the inclusion of the lac I gene

8. The Lac Operon in E. coli
With no proteins bound to the operator region, RNAP is free to bind to the promoter and transcribe the genes A P O
lacZ
lacY
lacA
pre-mRNA

9. The Lac Operon in E. coli
If there is no lactose present, this would be a waste of valuable resources like ATP and amino acids A P O
lacZ
lacY
lacA
pre-mRNA

10. The Lac Operon in E. coli
To prevent transcription, the genes can be “turned off” when a repressor protein is bound to the operator sequence
repressor A P O
lacZ
lacY
lacA
GET OUT OF THE WAY!

11. The Lac Operon in E. coli
When lactose is present, it will bind to the repressor, change its 3D shape and remove it from the operator. A P O
lacZ
lacY
lacA

12. The Lac Operon in E. coli
When lactose is present, it will bind to the repressor, change its 3D shape and remove it from the operator. A P O
lacZ
lacY
lacA

13. The Lac Operon in E. coli
RNAP is now free to bind the promoter and transcribe the sequence A P O
lacZ
lacY
lacA

14. The Lac Operon in E. coli
Since the presence of lactose removes the repressor and “turns on” the genes, lactose is called an inducer. A P O
lacZ
lacY
lacA
repressor
inducer

15. The Lac Operon in E. coli
Operon is OFF A P O
lacZ
lacY
lacA

16. The Lac Operon in E. coli
Operon is ON A P O
lacZ
lacY
lacA

18. The trp Operon in E. coli
If ingested levels of the amino acid tryptophan are low, e. coli is able to synthesize it on its own.
When tryptophan levels are low, the try operon is turned on to make the enzymes necessary to synthesize more.

19. The trp Operon in E. coli
Again, the cells should not waste resources making more tryptophan if it is already present

20. The trp Operon in E. coli O P No tryptophan = operon on
Enzymes are synthesized to make trp
trpE O P
trpD
trpC
trpB
trpA

21. The trp Operon in E. coli Tryptophan present = operon off
Trp is a co-repressor (needed for repressor to function)
repressor P O
trpE
trpD
trpC
trpB
trpA
co-repressor