Control Mechanisms

1. Control Mechanisms Gene Regulation

2. Gene Regulation • ~ 42,000 genes exist that code for proteins in humans • Not all proteins are required all the time • Example: Insulin only required when glucose level is high • Gene Regulation is a mechanism turning on or off of specific genes depending on the requirements of the organism • Gene regulation is vital to an organism’s survival

3. Housekeeping Genes • Genes that are switched on all the time because they are always needed in the cell for life functions • These genes are constantly transcribed and translated

4. Transcription Factors • Proteins that switch on genes by binding to DNA and helping the RNA polymerase to bind • Turn on genes when required

5. Four Leves of Control of Gene Expression In Eukaryotic Cells

8. The mRNA molecules undergo changes in the nucleus before translation occurs • Introns are removed and exons are spliced together

11. Two important regulatory mechanisms:

12. Operons • Cluster of genes, often functionally related, forming a tight cluster on the genome • Primarily occurring in prokaryotes (such as bacteria) • Also some in eukaryotes • Controlled by a common ‘ON/OFF’ switch • Operonsare under the control of regulatory elements and factors that bind to these elements. • What are these elements?

13. Operons

14. Operons • Transcription of the cluster results in a single molecule, • The molecule is a multi-gene transcript of mRNA • It codes for several proteins and is • directly translated into distinct protein products.

15. Operon Gene Regulation • Control of an operon is a type of gene regulation that enables organisms to regulate the expression of various genes depending on environmental conditions. • Operonregulation can be either negative or positive by induction or repression.

16. Operon Gene Regulation

17. Operon Gene Regulation • Induction and repression respond to specific substances, called effectors • Effectors control the activity of a specific set of genes

18. The lacOperon • The lacoperon is a cluster of genes of the model bacterium Escherichia coli • The first operon to be discovered • The lacoperon is regulated by several factors including the availability of glucose and lactose. • It consists ofa promoter, an operator, followed by a group of lactose-utilizing genes • Promoter: the binding site of RNA polymerase. • Operator: regulatory sequence that act as switch.

19. Lactose • A disaccharide found in milk or milk products • Consists of two sugars: glucose and galactose • E coli found on the intestinal lining of mammals can use the energy supplied by lactose for growth • To use the energy, E coli must split lactose into its two monomer sugars Galactose Glucose

20. Escherichia coli • E coli produces an enzyme to degrade lactose • Enzyme called β-galactosidase • There is no need for E coli to produce this enzyme at all times • It only produces the enzyme when lactose is present • E coli uses negative regulation to control the transcription and translation of the β-galactosidase gene

21. Negative control system

22. The lacOperon • The lactose-utilizing genes are: • lac Z, lac Y, and lac A • lac Z gene encodes the enzyme β-galactosidase • lac Y gene encodes β-galactosidasepermease(an enzyme that causes lactose to permeate the cell membrane and enter the cell) • lac A gene encodes a transacetylase (function unknown) lac Z lac Y lac A

23. Lacl Protein: • A repressor protein • Blocks the transcription of the β-galactosidase gene • It does that by binding to the Lactose operator and getting in the way of the RNA polymerase (Repressor protein: regulatory molecule binding to an operator site and preventing transcription of an operon)

24. When lactose is not present • The promoter and operator regions overlap • When the Lacl protein binds to the operator, it converts part of the promoter, which is the binding site for RNA polymerase

25. When lactose is present • The presence of lactose removes Lacl protein (repressor) • Therefore, lactose is known as the signal molecule or an inducer

27. Protein Transcription • Lactose binds to the Lacl protein changing the conformation of the Lacl protein • This change results in the inability of the new complex to stay bound to the operator region of the lacoperon • The complex falls off the DNA allowing RNA polymerase to proceed onward and transcribe the lacoperon

28. In the case of the lacOperon, the level of lactose is an effector, meaning that it controlsthe activity of a specific set of genes • The lacOperon is an example of enzymeinduction

29. Operon Gene Regulation

30. The trpOperon • Another example of coordinated regulation • In contrast to the lacOperon (transcription induced with presence of lactose), the trpOperon is repressed when high levels of tryptophan are present. • In this case, the effector is the level of tryptophan • Tryptophan is an amino acid that is used by E coli cells for the production of protein • E coli cells located on the intestinal lining of a mammal can absorb tryptophan from the mammal’s diet

31. The trpOperon • Consists of five genes • These five genes code for five polypeptides that make three enzymes needed to synthesize tryptophan

32. Corepressor • Since tryptophan itself is needed to inactivate the trpoperon, it is called a corepressor • Corepressor: - a molecule that binds to a repressor to activate it- usually the product of an operon

33. When Tryptophan Level is low • When the level of tryptophan is low, the shape of the trppressor protein changes • This is due to the lack of tryptophan corepressor • The trp repressor can no longer stay bound to the trp operator and it falls off • The RNA polymerase is free to transcribe the trpoperon genes • This results in an increase in tryptophan production

34. When Tryptophan Level is high • The amino acid tryptophan binds to the trp repressor protein, altering its shape • The trp-repressor-tryptophan complex can now bind to the trp operator • This way transcription is blocked

35. lacOperon • The lacl repressor protein binds to the operator when lactose levels are low • High level of lactose induce the operon • trpOperon • The corepressor tryptophan binds to the trp repressor protein, and the complex binds to the operator when tryptophan levels are high • High levels of tryptophan repress the operon

36. Comparison of the lacOperon and the trpOperon

37. Comparison of the lacOperon and the trpOperon

38. Comparison of the lacOperon and the trpOperon

39. Comparison of the lacOperon and the trpOperon

40. Comparison of the lacOperon and the trpOperon lacOperon trpOperon