1. Regulation of Gene Expression
Prokaryotes and Eukaryotes

2. Regulation of Gene Expression
A cell contains the entire genome of an organism– ALL the DNA.
Gene expression = transcribing and translating the gene
Regulation allows an organism to selectively transcribe (and then translate) only the genes it needs to.
Genes expressed depend on
the type of cell
the particular needs of the cell at that time.

3. Gene Regulation in Prokaryotes
Prokaryotes organize their genome into operons
Operon = a group of related genes
One promoter sequence at the very beginning
All of the genes will be transcribed together (in one long strand of RNA.

4. Question… What is the benefit of organizing the genome into operons?
It’s more efficient – transcribe everything you need for a process at once.

5. Repressible Operon: Trp Operon
Repressible Operon = Operon that is usually “ON” but can be inhibited
The Trp Operon
example of a repressible operon
Genes that code for enzymes needed to make the amino acid tryptophan

6. TrpR Gene TrpR gene is the regulatory gene for the Trp operon
Found somewhere else on the genome
NOT part of the Trp operon
TrpR gene codes for a protein = TrpR repressor
TrpR gene is transcribed and translated separately from the Trp operon genes.

7. TrpR Repressor Repressor protein is translated in an inactive form
Tryptophan is called a corepressor
When tryptophan binds to the TrpR repressor, it changes it into the active form

8. Operator Region
There is also an operator region of DNA in the Trp Operon
Just after the promoter region
The TrpR Repressor can bind to the operator if it’s in the active form

9. Trp Operon Transcription is “ON”
Occurs when there is no tryptophan available to the cell.
Repressor is in inactive form (due to the absence of tryptophan)
RNA Polymerase is able to bind to promoter and transcribe the genes.

10. Trp Operon Transcription is “OFF” Occurs when tryptophan is available
Tryptophan binds to the TrpR repressor  converts it to active form
TrpR protein binds to operator  blocks RNA Polymerase  no transcription

11. Question…
Under what conditions would you expect the trp operon to go from “OFF” to “ON” again?
When there is no longer tryptophan available– all of it has been used up

12. Inducible Operon: Lac Operon
Inducible operon = operon is usually “OFF” but can be stimulated/activated
Lac Operon
Example of an inducible operon
Genes code for enzymes that break down lactose

13. LacI gene LacI gene is the regulatory gene for the lac operon
Found somewhere else on the genome
NOT part of the lac operon
LacI gene codes for a protein = lacI repressor
LacI gene is transcribed and translated separately from the lac operon genes.

14. LacI Repressor
The lacI repressor protein is translated into an active form
When the lacI repressor is bound by lactose (also called allolactose) it becomes inactive
Lactose is the inducer

15. Lac Operon Transcription is “OFF”
When there is no lactose that needs to be digested
lacI repressor is in active form  binds to operator  blocks RNA Polymerase  no transcription

16. Lac Operon Transcription is “ON”
When there is lactose that needs to be digested
Lactose binds to lacI repressor  inactivates it
RNA Polymerase is able to bind to promoter  transcribe genes

17. Do all operons have operator regions?NO
There are some genes that always need to be transcribed  they do not need to have operators to regulate them in this manner.
Ex. genes that participate in cellular respiration

18. Positive Gene Regulation
In the lac operon there are other molecules to further stimulate transcription.
Lactose will only be digested for energy when there isn’t much glucose around
When glucose levels are low, level of cAMP molecule builds up

19. cAMP and CAP CAP = regulatory protein that binds to cAMP
CAP is inactive unless cAMP binds to it

20. Positive gene regulation
If there isn’t much glucose high levels of cAMP
CAP and cAMP bind  CAP can bind to the promoter  stimulates RNA Polymerase to bind

21. Positive gene regulation
When glucose levels rise again, cAMP levels will drop  no longer bound to CAP
CAP can’t bind to promoter  transcription slows down

22. Positive gene regulation
The lac operon is controlled on 2 levels:
Presence of lactose determines if transcription can occur
CAP in the active form determines how fast transcription occurs

23. Gene Regulation in Eukaryotes
Eukaryotes have large genomes
Other molecules have to help RNA Polymerase find the promoter and start transcription
Transcription factors
Enhancer sequences

24. Transcription Factors
Series of proteins that bind to the promoter to help RNA Polymerase bind
RNA Polymerase also has to bind transcription factors in order to be able to start transcription.
Like a sign on the door that helps direct you to a particular room in the school.

25. Question…
How might binding transcription factors help RNA Polymerase bind?
Creates an area that chemically attracts RNA Polymerase more

26. Enhancer sequences
Sequences of DNA that are far away from the gene they help transcribe
Process:
Activator molecules bind to the Enhancer sequence
Enhancer loops around so that the activators can also bind to the transcription factors
Together with RNA polymerase they all cause transcription to start
Enhancers can be up to 20,000 bp away from promoter. Like an even bigger sign to help direct people to a room

27. Cell-specific Regulation
Each cell has the DNA to transcribe any gene
Different activators and transcription factors in specific cells will determine which genes are transcribed  which proteins are translated