1. Introduction to Gene Expression

2. Gene Expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional protein.
Several steps in the gene expression process may be modulated, including:
Transcription
RNA Splicing (taking out Introns)
Translation
Post translational modification of a protein

4. Gene Expression

5. Constitutive Genes that are ALWAYS ON
Genes that specify cellular components that perform housekeeping functions
Structure is simpler
Example:
The ribosomal RNAs
Proteins involved in protein synthesis
Regulatory proteins

6. Non-Constitutive Genes: Inducible and Repressible Genes
Are expressed only as needed:
Their amount may vary depends on the condition
Their structure is relatively complicated with some response elements.
Why?
Would anabolic/ catabolic pathway have genes that are inducible or repressible?

7. Anabolic Pathway Repressible
Anabolism: Synthesis of molecules from simpler subunits
Example:
If an essential nutrient is not present in the environment, bacteria must be able to synthesize the nutrient on its own
However, bacteria do not need to produce the essential nutrient if it is already present in the environment

8. Catabolic Pathway Inducible
Catabolism: Breakdown of complex molecules into simpler units
Example:
It would be a waste of energy for bacteria to make enzymes for the breakdown of a substance if that substance does not exist

9. Control Mechanisms
There are 42,000 genes that code for proteins in humans
Not all proteins are required at all times
E.g. Insulin is only required in a cell when glucose levels are high
Regulation is therefore vital to an organism’s survival

10. Why turn genes on and off?
Cell specialization
Each cell of a multicellular eukaryote expresses only a small fraction of its genes
Development
Different genes needed at different points in life cycle of an organism
Responding to organism’s needs
Cells must be able to turn genes on and off in response to signals from their external and internal environment

11. Gene regulation Regulation of gene expression is of two types:
Positive regulation: When the expression of a gene is increased by the presence of specific regulatory element (aka Activator or positive regulator)
Negative regulation: When the expression of a gene is decreased by the presence of specific regulatory element (aka repressor )

14. Control of Gene Expression
Controlling gene expression is often accomplished by controlling transcription initiation.
Regulatory proteins bind to DNA to either block or stimulate transcription, depending on how they interact with RNA polymerase.

15. Control of Gene Expression
Prokaryotic organisms regulate gene expression in response to their environment.
Eukaryotic cells regulate gene expression to maintain homeostasis in the organism.

16. Regulatory Proteins
Gene expression is often controlled by regulatory proteins binding to specific DNA sequences.
regulatory proteins gain access to the bases of DNA at the major groove
regulatory proteins possess DNA-binding motifs

17. Regulatory Proteins
DNA-binding motifs are regions of regulatory proteins which bind to DNA
helix-turn-helix motif
homeodomain motif
zinc finger motif
leucine zipper motif

18. Helix-Turn-Helix Motif

19. Homeodomain Motif

20. Zinc Finger Motif

21. Leucine Zipper Motif

22. Principles of Gene Regulation
Most prokaryotic genes are regulated in units called Operons
Operon:
A group of genes that share a single promoter
Genes code for proteins that have related functions

23. Operon Components Promoter
Region on DNA where RNA Polymerase binds to start transcription
Acts as the on/off switch for genes
Operator
Region on DNA that controls the access of RNA Polymerase to the genes
Can activate or repress transcription
Structural genes
-Genes to be transcribed by RNA Polymerase

24. Operon Regulation Regulatory gene
Region on DNA that codes for the production of the regulatory protein
Upstream of the operon
Constitutive expression: transcribed continuously
Regulatory protein
Binds to perator and blocks attachment of RNA Polymerase
Allosteric: Alternates bewteen active and inacive forms

25. Example: Trp Operon Regulatory Gene: TrpR
Allosteric regulatory protein:
- Has 2 forms: Active or inactive

26. Operon Regulation
Effector molecule: Any molecule that can regulate the activity of a protein
Inducer: effector molecule that binds repressor protein to cause it to fall off operator
Corepressor: effector molecule that binds repressor protein to cause it to bind to the operator