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