Control Mechanisms: The lac operon and the trp operon .

1. Section 5.5 - Nelson Biology 12, pages 255-258 Control Mechanisms:The lacoperonand the trpoperon.

2. Genes & Proteins • Approximately 42, 000 genes exist that code for proteins in humans. • Not all proteins are required at all times. • Cells have developed methods by which they can control the transcription and translation of genes, depending on their need.

3. Why? • The genetic information encoded in the DNA strand is far from being the whole story. A simple set of protein blueprints would hardly be useful, because each of our cells would make all of the 42,000 proteins continually. • But brain cells don't need to make hemoglobin, and red blood cells don't need to make acetylcholine receptors. Each cell needs to be able to control the construction of its proteins so that it only builds the proteins needed for its own function. • To solve this problem, our DNA also contains a lot of regulatory information that specifies when and where each protein should be made. • Unlike the genetic information, this regulatory information is read without unwinding the DNA double helix. Instead, an army of regulatory proteins feels along the surface of the DNA double helix, reading the parts of the bases that are exposed and looking for the appropriate instructions. • Some of these proteins, when they find the appropriate instructions, bind to DNA and block the production of proteins that are encoded in the local area. Other regulators enhance the production of proteins, coaxing RNA polymerase to begin its function of transcribing messenger RNA. • The nucleus is a flurry of these regulatory proteins, as they control the production of proteins that are currently needed and block synthesis of proteins that are not.

4. HOUSE-KEEPING GENES • HOUSE KEEPING GENES – certain genes that are always needed in a cell. • They are constantly being transcribed and translated.

5. Transcription Factors • Transcription factors turn genes on when they are needed. • Transcription factors are proteins that switch on genes by binding to DNA and helping the RNA polymerase to bind.

6. Gene Regulation • Gene regulation is the turning on or off of specific genes depending on the requirements of an organism. • Gene regulation is vital to an organism’s survival.

7. Gene Regulation • Regulation of genes can occur at four levels. • Transcriptional • Posttranscriptional • Translational • Posttranslational

10. Why is it so important for organisms to be able to regulate the expression of their genes? All cells in a multicellular organism have the same set of genetic material (DNA). However, they are specialized in different functions, e.g. different organs, and have to express the right amounts of different proteins in the right places. Furthermore, different proteins are needed during the different stages in the cell cycle, for example when cells are dividing or replicating their DNA. Bacteria do not differentiate into different cell types, but they need to adjust to their environment, depending on which food source is available (e.g. different kinds of sugar), starvation, temperature etc. Like eukaryotes, they also need to regulate the expression of certain genes during different cell cycle stages.

11. The IacOperon A specific example

12. What is lactose? • Lactose is a disaccharide found in milk/milk products that consists of two sugars: • Glucose • Galactose

13. Lactose:

14. E.Coli & Lactose • E. Coli bacterial cells found on the intestinal lining of mammals can use the energy supplied by lactose for growth. • To use the energy, E.coli must split the lactose in its two monomer sugars. • Galactose • Glucose

15. B-Galactosidase degrades lactose • B-galactosidase is the enzyme responsible for the degradation of lactose. When is a good time to produce B-Galactosidase?

16. What is an Operon? • The gene for B-galactosidase is part of an operon. • A prokaryote uses an operon to regulate genes and their respective products. • An operon is a cluster of genes under the control of one promoter and one operator in prokaryotic cells • It acts like a simple regulatory loop.

17. Lac Operon • The lacoperonconsists of a regulatory sequence and a cluster of 3 structural genes that code for proteins involved in the metabolism of lactose. • lacZ • lacY • lacA

20. One More Key Player: LacI- the Repressor Protein • LacI protein is a repressor protein • LacI blocks the transcription of the B-galactosidasegene • How? • by binding to the lactose operator and getting in the way of the RNA polymerase.

21. LacI Protein = Repressor Protein • The promoter and operator regions overlap; when the LacI protein binds to the operator, it covers part of the promoter, the binding site for RNA polymerase.

22. Road Block!!

23. Lac1 Protein = A ROAD BLOCK • The road block ensures that if no lactose is present in the cell’s environment, the lacoperon genes are NOT transcribed and translated into their respective proteins.

24. Brilliance: • LacI is a tetramer of four identical subunits that normally binds tightly to a specific region in the bacterial DNA, termed the operator, that is next to a region that encodes three lactose-metabolizing proteins. When bound there, it blocks production of the proteins. But when lac repressor binds to lactose and similar sugars, it changes shape and no longer can bind to the DNA. Then, RNA polymerase is free to transcribe the gene, and the proteins are made. • Notice what this accomplishes for the bacterial cell!! When lactose is rare, the lacoperon proteins are not made, because they are not needed. But when the bacterium stumbles upon a source of lactose, the plentiful sugars bind to lac repressor and force it to allow production of the enzymes, which quickly begin using the sugars for energy. When the source is depleted, lac repressor loses its bound sugars, and goes back to blocking the production of the proteins since they are no longer needed.

25. Level of Lactose = an Effector • The level of lactose is an effector. • The level of lactose controls the activity of a specific set of genes.

26. If lactose is present then the road block is removed. • If lactose is introduce into a cell’s environment, the roadblock must be removed to ensure that B-galactosidase is manufactured. • The presence of lactose removes the roadblock. • “signal molecule” • “inducer”

27. IacI Repressor Forms

28. How does it work? • Lactose binds to the LacI protein, which changes the shape (conformation) of the LacI protein. • The new complex (lactose and LacI) can’t stay bound to the operator region of the lacoperon. • The complex falls off the DNA, allowing the RNA polymerase to proceed onward and transcribe the lacoperon.

29. Summary:

30. Summary: TRANSCRIPTION IS INDUCED WHEN LACTOSE IS PRESENT!!

31. The trpOperon A co-repressor

32. What is tryptophan? • Tryptophan is an amino acid that is used by E.Colifor the production of protein. • E. Coli cells located on the intestinal lining of mammals can absorb tryptophan from the mammal’s diet. • If this source is not available, E.Coli must manufacture their own tryptophan. Tryptophan

33. If a dietary source of tryptophan is not available, E.Coli must manufacture their own tryptophan.

34. Tryptophan Levels • Once a high concentration of tryptophan is present in the cell, the genes for tryptophan production are no longer transcribed. E.Coli

35. The trpoperon • The trpoperon is a cluster of genes in a prokaryotic cell that is under the control of one promoter and one operator. • The genes govern the synthesis of the necessary enzymes required to synthesize the amino acid tryptophan.

36. The trpoperon… • The typoperon consists of 5 genes. • These 5 genes code for 5 polypeptides that make 3 enzymes needed to synthesize tryptophan.

37. TrpOperon… • When tryptophan levels are high, tryptophan binds to the trp repressor protein, altering its shape. • The trp repressor-tryptophan complex can now bind to the trp operator.

38. Co-repressor • Since tryptophan itself is needed to inactive the trpoperon, it is called a corepressor. • A corepressor is a molecule (usually the product of an operon) that binds to a repressor to activate it.

39. Tryptophan levels = effector • Trpoperon is repressed when high levels of tryptophan is present. • The effector is the level of tryptophan.

40. Low Levels of Tryptophan • When tryptophan levels decrease, the shape of the trp repressor protein changes back because of the lack of the tryptophan corepressor.

41. Low Levels of Tryptophan • 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, resulting in an increase in tryptophan production.

42. Summary of trpOperon:

43. Summary of Today’s Ideas:

44. Control Mechanisms • Operons are gene regulation mechanisms found in prokaryotes. • This strategy allows prokaryotes to regulate the level of protein synthesis or end product in a pathway.

45. The lacoperonis an example of enzyme induction. • The trpoperonis an example of enzyme repression. • Induction and repression are precise regulatory mechanisms that respond to specific substances, called effectors, that control the activity of a specific set of genes. • The effector in the lacoperon is the level of lactose, whereas the effector in the trpoperon is the level of tryptophan.

46. Lac Operon Summary • It regulates the production of B-galactosidase and other proteins involved in the metabolism of lactose. • It consists of a cluster of 3 genes under the control of 1 promoter and one operator. • The LacI repressor protein binds to the operator when lactose levels are low. • High levels of lactose induce the operon.

47. The lac OPERON:

48. The lac OPERON:

49. TrpOperon Summary • The trpoperon regulates the production of the amino acid tryptophan. • It consists of a cluster of 5 genes under the control of 1 promoter and 1 operator. • The co-repressor 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.

50. The trp OPERON: