Regulation of gene expression in prokaryotes

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Regulation of gene expression in prokaryotes

  1. 1. Regulation of Gene Expression in Prokaryotes Masih Sherafatian Copyright © 2009 Pearson Education, Inc.
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  8. 8. Prokaryotes Regulate Gene Expression in Response to Environmental Conditions •  Many Prokaryotic Genes Are Clustered and Regulated in Operons Copyright © 2009 Pearson Education, Inc.
  9. 9. Lactose Metabolism in E. coli Is Regulated by an Inducible System •  Genes coding for the primary structure of an enzyme are called structural genes. Copyright © 2009 Pearson Education, Inc.
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  15. 15. Constitutive Mutations •  Enzymes are produced regardless of the presence or absence of lactose •  lacI – : repressor gene •  lac OC : operator region Copyright © 2009 Pearson Education, Inc.
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  22. 22. The major predictions to be tested (1) the I gene produces a diffusible product (that is, a trans-acting product) (2) the O region is involved in regulation but does not produce a product (it is cis-acting); and (3) the O region must be adjacent to the structural genes in order to regulate transcription. Copyright © 2009 Pearson Education, Inc.
  23. 23. Genetic Proof of the Operon Model •  Creation of partially diploid bacteria •  The use of F’ plasmid makes it possible, for example, to introduce an I+ gene into a host cell whose genotype is I– , or to introduce an O+ region into a host cell of genotype OC. •  Mutation, IS, “superrepressed,” cannot interact with the inducer, lactose Copyright © 2009 Pearson Education, Inc.
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  26. 26. The Catabolite-Activating Protein (CAP) Exerts Positive Control over the lac Operon Copyright © 2009 Pearson Education, Inc.
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  31. 31. Predict the level of genetic activity of the lac operon as well as the status of the lac repressor and the CAP protein under the cellular conditions listed in the accompanying table Copyright © 2009 Pearson Education, Inc.
  32. 32. (a) With no lactose and no glucose, the operon is off because the lac repressor is bound to the operator and although CAP is bound to its binding site, it will not override the action of the repressor. (b) With lactose added to the medium, the lac repressor is inactivated and the operon is transcribing the structural genes. With no glucose, the CAP is bound to its binding site, thus enhancing transcription. (c) With no lactose present in the medium, the lac repressor is bound to the operator region, and since glucose inhibits adenyl cyclase, the CAP protein will not interact with its binding site. The operon is therefore “off.” (d) With lactose present, the lac repressor is inactivated; however, since glucose is also present, CAP will not interact with its binding site. Under this condition transcription is severely diminished and the operon can be considered to be “off.” Copyright © 2009 Pearson Education, Inc.
  33. 33. Crystal Structure Analysis of Repressor Complexes Has Confirmed the Operon Model Copyright © 2009 Pearson Education, Inc.
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  36. 36. The Tryptophan (trp) Operon in E. coli Is a Repressible Gene System •  This operon is regulated by two mechanisms •  Tryptophan is Corepressor •  constitutive mutations: trpR– Copyright © 2009 Pearson Education, Inc.
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  42. 42. Riboswitches Utilize Metabolite- sensing RNAs to Regulate Gene Expression •  mRNA sequences (or elements), present in the 5’-untranslated region (5’-UTR) upstream from the coding sequences. •  capable of binding with small molecule ligands, such as metabolites, •  creating a terminator structure. Copyright © 2009 Pearson Education, Inc.
  43. 43. The two important domains within a riboswitch •  Aptamer: binds to the ligand •  Expression platform: capable of forming the terminator structure Copyright © 2009 Pearson Education, Inc.
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  45. 45. The ara Operon Is Controlled by a Regulator Protein That Exerts Both Positive and Negative Control Copyright © 2009 Pearson Education, Inc.
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  50. 50. In a theoretical operon, genes A, B, C, and D represent the repressor gene, the promoter sequence, the operator gene, and the structural gene, but not necessarily in the order named. This operon is concerned with the metabolism of a theoretical molecule (tm). From the data provided in the accompanying table, first decide whether the operon is inducible or repressible. Then assign A, B, C, and D to the four parts of the operon. Explain your rationale. (AE = active enzyme; IE = inactive enzyme; NE = no enzyme) Copyright © 2009 Pearson Education, Inc.
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  52. 52. The system is inducible. C codes for the structural gene. B must be the promoter. The A locus is the operator, and the D locus is the repressor gene. Copyright © 2009 Pearson Education, Inc.

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