Lac operon sohil

Biochemistry MBBS

1. Operon Concept

2. Contents 1> INTRODUCTION 2> PRINCIPAL OF GENE REGULATION 3> CONCEPT OF GENE REGULATION 4> OPERON MODEL 5> GENE REGULATION IN EUKARYOTES Lac operon Tryptophan operon

3. What is gene expression? Synthesis of protein under the influence of gene is called gene expression All genes of cell are not expressed at all the time for example any hormone ( Eg Insulin)

4. It is a combined process of the transcription of a gene into mRNA , the processing of that mRNA and its translation into protein The gene is expressed in the form of protein Prokaryotic gene is controlled mainly at the level of transcription m-RNA synthesis

5. Regulation of gene expression Regulation of gene expression is absolutely essential for the growth, development and the very existence of an organism They are of two type :- 1.Positive Regulation (Inducers) 2.Negative Regulation (Repressors)

6. Types of Gene There are two types of genes - 1 .constitutive gene -they are present in almost each and every cell , therefore also know as housekeeping genes Example- citric acid cycle 2. Inducible gene – they are synthesized when molecular signals are given Example – Tryptophan pyrrolase of liver is induced by tryptophan Inducible genes are subject to regulated expression

7. One cistron – one subunit concept The chemical product of gene expression is a protein which may be an enzyme Each gene codes for specific enzyme- One gene –one enzyme , where as several enzymes are composed of more than two non-identical subunits The cistron is the smallest unit of genetic expression .It is the fragment of DNA coding for the subunit of a protein molecule. Thus the original concept one gene-one enzyme is replaced by one cistron -one subunit

8. Prokaryotes have a simple mechanism for coordinating the regulation of gene .In prokaryotes the gene are clustered on chromosome and transcribed together In prokaryotes the gene involved, in metabolic pathway are often present in a linear fashion called an operon Example – Lactose operon

9. OPERON Operon is a functioning unit of DNA containing a cluster of genes under the control of single promoter Its Unit of gene expression, which includes structural genes, control elements, regulator/inhibitor gene, promoter and operator area

10. LACTOSE OR LAC OPERON

11. Schematic Diagram of LAC OPERON

12. Lac operon

13. A i P O Z Y A RNA polymerase

14. A i P O Z Y A RNA polymerase

15. A i P O Z Y A RNA polymerase Galactosidase

16. A i P O Z Y A RNA polymerase Galactosidase

17. A i P O Z Y A RNA polymerase Galactosidase Permease

18. A i P O Z Y A RNA polymerase Galactosidase Permease

19. A i P O Z Y A RNA polymerase Galactosidase Permease Transacetylase

20. A i P O Z Y A RNA polymerase Galactosidase Permease Transacetylase

21. Regulation of LAC operon Type of regulation Regulated by Lactose Glucose Negative regulation Lac repressor Absent Present Positive regulation Induction of expression Present Absent Positive control Catabolite repression Present Present

22. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A active

23. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein

24. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein

25. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein

26. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein RNA polymerase

27. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein RNA polymerase

28. Negative regulation by Lac repressor (Glucose present, lactose absent) A i P O Z Y A Repressor protein RNA polymerase

29. Negative regulation by Lac repressor (Glucose present,lactose absent) A i P O Z Y A Repressor protein

30. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose

31. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose

32. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose

33. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose

34. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose

35. Positive regulation by induction of expression (Glucose absent,lactose present) A i P O Z Y A Repressor protein RNA polymerase Lactose Galactosidase Permease Transacetylase

36. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose glucose glucose glucose

37. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose glucose glucose glucose

38. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP

39. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP

40. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP

41. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP

42. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP

43. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP

44. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP Galactosidase Permease Transacetylase

45. Positive regulation by Catabolite repression (Glucose present,lactose present) A i P O Z Y A RNA polymerase Lactose Lactose Lactose Lactose Lactose cAMP CAP Galactosidase Permease Transacetylase

46. Lactase in human intestine is an inducible enzyme. Clinical manifestations of lactase deficiency and lactose intolerance are described. Examples of derepression in human beings: Induction of tryptophan pyrrolase by tryptophan Transaminases by glucocorticoids ALA synthase by barbiturates Glucuronyl transferase by barbiturates. Clinical Applications of Derepression

47. Repression is the mechanism by which the presence of excess product of a pathway shuts off the synthesis of the key enzyme of that pathway. Heme synthesis is an example. It is regulated by repression of ALA synthase, the key enzyme of the pathway. Regulation of Genes by Repression

48. Repression of heme synthesis by heme. (ALA: aminolevulinic acid; RNAP: RNA polymerase)

49. Tryptophan operon

50. Tryptophan operon TRP Operon is a group of genes encode biosynthetic enzyme for amino acid Trp Found in E Coli. Repressible Operon Eg of Feedback Inhibition Trp Operon expressed on- when trp level is low Repressed when- trp level is high

51. Tryptophan operon

52. content TRP operon consists of: 5 structural genes: Trp e Trp d Trp C Trp B Trp a Regulatory Gene Promoter Operator Terminator

53. Turning on and turning off Trp operon is normally ON. If surrounding medium contains Large amount of Trp : Switched OFF. Trp repressor regulates Synthesis of Trp

54. Mechanism of on and off Trp repressor is bind with DNA of operator and block the transcription of structural gene. Trp repressor doesn’t always bind to DNA instead it bind and block Transcription only when Trp is present When Trp is around it, attaches to the repressor molecule and change their structural Configuration Small Molecule like Trp Which Switches a repressor into its Active state is called Co-represser Trp repressor(inactive) + Co-represser = Active repressor. Active repressor bind with Operator to block Transcription

55. Attenuation Mechanism based on Coupling of Transcription and Translation. Trp operon is a classical example of Attenuation. Attenuator Region: Additional control site for regulation of transcription

56. Continued… If any of RNA polymerase escape repression at the operator, transcription begins at the Promoter. But, due to presence of TRP a region, transcription is prematurely terminated at Attenuter region In absence of TRP, attenuter has no effect on transcription and Transcription proceed as usual

57. GENE EXPRESSION IN EUKARYOTES The important features of eukaryotic gene expression along with the regulatory aspects are described in the eukaryotic cells also employ variety of other mechanisms to regulate gene expression. The most important ones are listed below , and briefly described next

58. Continued… 1. Gene Amplification 2.Gene Rearrangment 3.Processing of RNA 4. Alternate mRNA splicing 5.Transport of mRNA from nucleus to cytoplasm 6.Degradation of mRNA

59. Gene Amplification Methotrexate resistance by cancer cells Methotrexate inhibits the enzyme dihydrofolate reductase . The malignant cells develop drug resistance to long term administration of methotrexate by amplifying the genes coding for dihydrofolatereductase Result for active folate more replication

60. Gene rearrangement

61. Processing of mRNA Splicing Capping Poly (A) Tail

62. Alternate mRNA splicing For example, tropomyosin (TM) is an actin filament-binding protein that regulates the functions of actin in both muscle and nonmuscle cells.

63. Degradation of mRNA Prevented by Capping and poly (A) Tail Also by Stem loop in Non coding region Certain hormone acts by acting on protein that regulates formation of above factors

64. Co and Post transcriptional regulation-mRNA Editing apoprotein ( apo ) B—an essential protein component of chylomicrons and very low density lipoproteins. Apo B mRNA is made in the liver and the small intes - tine; however, in the intestine only, the C residue in the codon (CAA) for glutamine is deaminated to U, changing the sense codon to a nonsense or stop codon ( UAA) This results in a shorter protein ( apo B-48, representing 48% of the message) being made in the intestine (and incorporated into chylomicrons) than is made in the liver ( apo B-100, full-length, incorporated into VLDL)

65. ApoB48

66. Trans acting regulation – locally such as TFs

67. Cis Acting regulation- Hormone ( Not localised )

68. Cis Acting regulation- Hormone ( Not localised )

69. Impacting mRNA stability

70. Impacting mRNA stability

71. At translation level Phosphorylation of eIF-2 inhibits its function and so inhibits translation at the initiation step

72. Post Translationally Hypermethylation inhibits gene expression