1. Agenda 3/16 Genes Expression Warm Up Prokaryotic Control Lecture
Operon Practice
Homework: Chp 15 notes (Quiz Tuesday), Video and Notes, Finish Lac Operon Worksheets
Turn in: Nothing

2. Warm Up
Why do we need to control when proteins are made or not made?

3. Control of Prokaryotic
Gene Expression
Prokaryotic regulation is different from eukaryotic regulation.
Prokaryotic gene regulation tends to be negative. Meaning that the gene is usually activated unless some regulator inhibits it or deactivates it.
Eukaryotic gene regulation is usually positive. Meaning that the gene is usually deactivated unless a regulator activates it or turns it “on”.

4. Unique to Eukaryotes
Eukaryotic cells have many more genes (i.e. 23,000 in human cells) in their genomes than prokaryotic cells (i.e. average 3000).
Physically there are more obstacles to regulate eukaryotic genes because there is so much more DNA to manage. For example, eukaryotic chromatin is wrapped around histone proteins.
In addition there are other nonhistone proteins that are used in eukaryotic gene expression that are not used in prokaryotic gene expression.

5. Prokaryotic Regulation of Genes
Regulating Tryptophan Synthesis.
Why make it when you don’t need it??
Produce something that will interfere with the function of the enzyme in the pathway.
Produce a gene regulator that can inhibit the transcription of one biochemical pathway enzymes.
In all cells, there are genes that code for proteins. It would be wasteful if genes were expressed when there is not a need for the protein product.
Example - why make enzymes used to break glycogen into glucose if the cell has a reservoir of glucose molecules? OR if the cell has no glycogen?
Regulating Prokaryotic Genes
Prokaryotic regulation is different from eukaryotic regulation.
Prokaryotic gene regulation tends to be negative. Meaning that the gene is usually activated unless some regulator inhibits it or deactivates it.
Eukaryotic gene regulation is usually positive. Meaning that the gene is usually deactivated unless a regulator activates it or turns it “on”.

6. Operon and Prokaryotic Gene Expression
Operon- A group of prokaryotic genes with a related function that are often grouped and transcribed together. In addition, the operon has only one promoter region for the entire operon.

7. Operon and Prokaryotic Gene Expression
An operon is composed of the following:
Structural genes- genes that are related and used in a biochemical pathway. (enzymes, proteins etc.)
Promoter-The nucleotide sequence that can bind with RNA polymerase to start transcription. This sequence also contains the operator region.
Operator-The nucleotide sequence that can bind with repressor protein to inhibit transcription.

8. Regulator Genes and Repressors
Regulator gene- This gene produces a protein called a repressor that can inhibit the transcription of an operon by attaching to the operator.

9. Interaction of Modulators and Repressors
Repressors have allosteric properties.
Modulators can bind to the repressor at an allosteric site changing the conformation of the repressor
This results in activating or deactivating the repressor.
Prokaryotic Gene Expression
Prokaryotic genes with related function are often grouped together. This grouping is called an operon. All the related genes will be transcribed together. In addition, the operon has only one promoter region for the entire operon. An operon is composed of the following:
-Structural genes- genes that are related and used in a biochemical pathway.
-Regulator gene- This gene will produce a repressor protein that will turn the operon off or inhibit the transcription of the operon. Repressors have allosteric properties. Modulators can bind to the repressor at an allosteric site changing the conformation of the protein. The modulator is usually a product of the biochemical pathway.

10. Lactose and the Inducible lac Operon
Negative Gene Regulation
1. Inducible operon- the lac operon. This operon has the ability to convert lactose into glucose and galactose. This involves three structural genes
Inducible Operon-Is an operon that is induced, or turned on, by a particular modulator that inhibits the repressor. Inhibiting the repressor will allow for the transcription of the operon. i.e. inducible operons are “off” unless something turns them “on” – inducing them to act.
Example lac operon
The products of the lac operon produces enzymes necessary to break down lactose into glucose and galactose. This allows the prokaryote to use the disaccharide lactose as an energy source when necessary. There is no need to use this gene unless lactose is present, so the gene is “off” unless lactose induces it to be “on.”
Three structural genes of the lac operon
lacZ-makes b-galactosidase an enzyme that breaks lactose into glucose and galactose
lacY-makes permease which increases the cell's permeability for lactose.
lacA-makes transacetylase whose function is unknown.
The lac operon is an example of an inducible operon.
Usually off until needed

11. Animation of the lac Operon and Presence of Lactose

12. Absence of Lactose and the lac Operon
If no lactose is present, the repressor protein is active, binding to the operator site. This prohibits the RNA polymerase from transcribing the operon.

13. Animation of the lac Operon and Absence of Lactose

14. Synthesis of Tryptophan and the Repressible trp Operon
The purpose of the trp operon is to synthesize the amino acid tryptophan when needed.
Unlike inducible operons, repressible operons are “on” unless the presence of something actively turns them “off” – represses them to make them inactive.
Repressible operon-the trp operon. This is a repressible operon because it is “on” unless excess tryptophan is present. When the cell does not have tryptophan, the repressor enzyme is inactive. RNA polymerase can attach to the promoter region and transcribe the genes necessary for the synthesis of tryptophan. When tryptophan is present, however, it represses the operon or inhibits the transcription of the operon it by activating the repressor protein. The trp operon includes five structural genes that make five enzymes necessary for the synthesis of tryptophan. The repressor protein that controls this operon is inactive without tryptophan present. Tryptophan is called a corepressor as it attaches to the repressor.
-The purpose of the trp operon is to synthesize the amino acid tryptophan when needed.
-Unlike inducible operons, repressible operons are “on” unless the presence of something actively turns them “off” – represses them to make them inactive.

15. Compare and contrast inducible and repressible operons

16. Animation of the trp Operon and Absence of Tryptophan

17. Tryptophan Present and the Repressible trp Operon
The repressor protein becomes active when tryptophan is present. The repressor protein binds to the operator region blocking RNA polymerase from attaching to the promoter region.
Why does this make sense?

18. Animation of the trp Operon and Presence of Tryptophan

19. Lac and trp Operons-Examples of Negative Gene Regulation
The lac and typ operons are example of negative gene regulation as the repressor protein inhibits transcription of the operons.

20. Example of Positive Gene Regulation
While most prokaryotic gene regulation is negative, there are some examples of positive gene regulation. The lac operon has both a negative and a positive way to regulate the gene. The purpose of the lac operon is to break down lactose when lactose is present and when glucose is absent. The way the cell senses whether or not it has glucose involves another regulatory gene. This produces cAMP receptor protein (CRP). The CRP binds to a site next to the promoter region making it easier for the RNA polymerase to attach. The CRP is an allosteric protein. It becomes active when cAMP binds to it. cAMP accumulates in the cell when glucose is absent.

21. More details than you need….
The lac operon has both a negative and a positive way to regulate the gene.
The purpose of the lac operon is to break down lactose when lactose is present and when glucose is absent.
The way the cell senses whether or not it has glucose involves another regulatory gene. it.
cAMP accumulates in the cell when glucose is absent.

22. Both Lactose and Glucose Present
Lactose present, glucose present (cAMP level low), little lac mRNA synthesized