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The Operon 操縱元

Learn about operons, functional units of genomic material, and how they regulate gene expression. Discover the mechanisms behind negative and positive regulation, as well as the role of repressor proteins and transcription factors. Explore the lac operon and understand inducers and repressors. Dive into cis-acting and trans-acting mutations and their effects on gene expression. Gain insights into operons in eukaryotes, including humans.

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The Operon 操縱元

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  1. The Operon 操縱元 a functioning unit of genomic material containing a cluster of genes under the control of a single regulatory signal or promoter

  2. Ex Biochem c12-operon 12.1 Introduction Figure 12.1

  3. Ex Biochem c12-operon 12.2 Regulation Can Be Negative or Positive • In negative regulation, a repressor protein binds to an operator to prevent a gene from being expressed. Figure 12.2

  4. Ex Biochem c12-operon 12.2 Regulation Can Be Negative or Positive • In positive regulation, a transcription factor is required to bind at the promoter. • This enables RNA polymerase to initiate transcription. • Enhancer, activator Figure 12.3

  5. Ex Biochem c12-operon 12.3 Structural Gene Clusters Are Coordinately Controlled • Genes coding for proteins that function in the same pathway may be: • located adjacent to one another • controlled as a single unit that is transcribed into a polycistronic mRNA Figure 12.4

  6. Ex Biochem c12-operon 12.4 The lac Genes Are Controlled by a Repressor • Transcription of the lacZYA gene cluster is controlled by a repressor protein. • The repressor binds to an operator that overlaps the promoter at the start of the cluster. • The repressor protein is a tetramer of identical subunits coded by the gene lacI. Figure 12.5

  7. Ex Biochem c12-operon 12.5 The lac Operon Can Be Induced • Small molecules that induce an operon are identical with or related to the substrate for its enzymes. • β-galactosides are the substrates for the enzymes coded by lacZYA. • In the absence of β-galactosides, the lac operon is expressed only at a very low (basal) level. • Addition of specific β-galactosides induces 誘發 transcription of all three genes of the operon. • The lac mRNA is extremely unstable; • as a result, induction can be rapidly reversed. • The same types of systems that allow substrates to induce operons coding for metabolic enzymes can be used to allow end-products to repress the operons that code for biosynthetic enzymes.

  8. Ex Biochem c12-operon Figure 12.06: lac expression responds to inducer.

  9. Ex Biochem c12-operon 12.6 Repressor Is Controlled by a Small Molecule Inducer • An inducer functions by converting the repressor protein into a form with lower operator affinity. • Repressor has two binding sites: • one for the operator • one another for the inducer • Repressor is inactivated by an allosteric interaction: • Binding of inducer at its site changes the properties of the DNA-binding site

  10. Ex Biochem c12-operon Inducer of lac Operon • IPTG: common inducer for lac Operon used in lab • Similar structure to lactose • Although can NOT be digested by beta-galactosidase

  11. Ex Biochem c12-operon Figure 12.07: A repressor tetramer binds the operator to prevent transcription. Figure 12.08: Inducer inactivates repressor allowing gene expression. http://www.youtube.com/watch?v=oBwtxdI1zvk

  12. Ex Biochem c12-operon 12.7 cis-Acting Constitutive Mutations Identify the Operator • Mutations in the operator cause constitutive expression of all three lac structural genes. • These mutations are cis-acting and affect only those genes on the contiguous 連續的 stretch of DNA. • Cis-acting • Referring to a regulatory sequence in DNA (e.g., enhancer, promoter) that can control a gene only on the same chromosome. • In bacteria, cis-acting elements adjacent or proximal to the genes they control, whereas in eukaryotes they may also be far away Figure 12.9

  13. Ex Biochem c12-operon 12.8 trans-Acting Mutations Identify the Regulator Gene • Mutations in the lacI gene: • are trans-acting • affect expression of all lacZYA clusters in the bacterium • trans-acting • Referring to DNA sequences encoding diffusible proteins (e.g., transcription activators and repressors) that control genes on different chromosomes • Mutations that eliminate lacI function: • cause constitutive expression • are recessive

  14. Ex Biochem c12-operon 12.8 trans-Acting Mutations Identify the Regulator Gene • Mutations in the DNA-binding site of the repressor are constitutive because the repressor cannot bind the operator. • Mutations in the inducer-binding site of the repressor: • prevent it from being inactivated • cause uninducibility • Mutations in the promoter are: • uninducible • cis-acting Figure 12.10

  15. Ex Biochem c12-operon 12.14 Repressor Protein Binds to the Operator • Repressor protein binds to the double stranded DNA sequence of the operator. • The operator is a palindromic sequence of 26 bp. Figure 12.17

  16. Ex Biochem c12-operon Operons in eukaryotes • gene order in eukaryotes is NOT random. • numerous reports of gene clusters of related function in eukaryotes, even humans • significant tendency for genes from the same metabolic pathway to cluster. • Extensive clustering of non-homologous genes that are co-ordinately expressed in eukaryotes, including humans

  17. Ex Biochem c12-operon Operons in eukaryotes • At the functional level, physical clustering may be advantageous because it allows groups of genes to be co-ordinately regulated at the levels of nuclear organization and/or chromatin. • The alleles could interact well by being co-localized in regions of chromosomes that facilitate co-ordinate regulation

  18. Ex Biochem c12-operon Hurst, 2004

  19. Ex Biochem c12-operon Osbourn, 2009

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