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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 1 Ex Biochem c12-operon 12.1 Introduction Figure 12.1 2 12.2 Regulation Can Be Negative or Positive Ex Biochem c12-operon 3 In negative regulation, a repressor protein binds to an operator to prevent a gene from being expressed. Figure 12.2 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 Ex Biochem c12-operon 4 Ex Biochem c12-operon 12.3 Structural Gene Clusters Are Coordinately Controlled Genes coding for proteins that function in the same pathway may be: Figure 12.4 located adjacent to one another controlled as a single unit that is transcribed into a polycistronic mRNA 5 12.4 The lac Genes Are Controlled by a Repressor Ex Biochem c12-operon 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 6 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. 7 Ex Biochem c12-operon Figure 12.06: lac expression responds to inducer. 8 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: 9 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 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 betagalactosidase 10 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 11 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 Figure 12.9 12 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 Ex Biochem c12-operon 12.8 trans-Acting Mutations Identify the Regulator Gene Mutations in the lacI gene: trans-acting are trans-acting affect expression of all lacZYA clusters in the bacterium 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 13 Ex Biochem c12-operon 12.8 trans-Acting Mutations Identify the Regulator Gene Mutations in the DNAbinding site of the repressor are constitutive because the repressor cannot bind the operator. Mutations in the inducerbinding site of the repressor: Figure 12.10 14 prevent it from being inactivated cause uninducibility Mutations in the promoter are: uninducible cis-acting 12.14 Repressor Protein Binds to the Operator Figure 12.17 Ex Biochem c12-operon Repressor protein binds to the double stranded DNA sequence of the operator. The operator is a palindromic sequence of 26 bp. 15 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 16 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 colocalized in regions of chromosomes that facilitate co-ordinate regulation 17 Ex Biochem c12-operon Hurst, 2004 18 Ex Biochem c12-operon 19 Osbourn, 2009