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Gene Regulation, Part 1 Lecture 15 Fall 2008. Metabolic Control in Bacteria Regulate enzymes already...
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Transcript of Gene Regulation, Part 1 Lecture 15 Fall 2008. Metabolic Control in Bacteria Regulate enzymes already...
Gene Regulation, Part 1Lecture 15Fall 2008
Metabolic Control in Bacteria
• Regulate enzymes already present– Feedback Inhibition– Fast response
• Control production of enzymes– Regulates transcription– Longer-term response
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Fig. 18.2
Metabolic Control in Bacteria
• Operon– Unit of genetic function, found in bacteria and
phages, consisting of an operator, promoter & coordinately regulated cluster of genes whose products function in a common pathway
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Metabolic Control in Bacteria• Promoter
– Specific area of DNA that designates the start of a gene
– Where RNA polymerase binds• Operator
– Sequence of nucleotides near the start of an operon to which an active repressor can attach
– On-off “switch” for the cluster of genes
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Fig. 18.3
Metabolic Control in Bacteria• Regulatory gene
– Gene that codes for a protein that controls the transcription of another gene or group of genes
• Repressor– Protein that inhibits gene transcription– Binds to operator & prevents RNA polymerase from
attaching to promoter
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Fig. 18.3
Metabolic Control in Bacteria
• Repressors – Specific to an operator of a specific gene
• Regulatory genes expressed continuously, but at low rate– On-off regulated by concentration of
repressors– Repressors are allosteric proteins
• Active & inactive form• Corepressors
– Small molecules that cooperate with repressor to turn off transcription
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Metabolic Control in Bacteria
• Repressible Operon– Operon where transcription is normally “on”,
but that can be inhibited (repressed)– E.g., trp operon (tryptophan)
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Fig. 18.3
Metabolic Control in Bacteria
trp operon• trp repressor synthesized in an inactive form• Tryptophan acts as corepressor
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Fig. 18.3
Metabolic Control in Bacteria
• Inducible operon– Operon where transcription is normally “off”, but that
can be stimulated (induced)– E.g., lac operon (lactose)
• lac repressor synthesized in active form
• Binds to operator• Prevents RNA
Polymerase from binding
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Fig. 18.4
Metabolic Control in Bacteria
• Inducer binds to lac repressor• Inducer: specific small molecule that binds to repressor
protein and changes its shape
– Repressor inactivated– Releases/does not bind with operator– Transcription occurs
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Fig. 18.4
Negative and positive gene regulation
• Negative gene regulation– Operons switched off by active form of a
repressor protein– E.g., lac and trp operons
• Positive gene regulation– Regulatory protein interacts directly with the
genome to switch transcript on
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Negative and positive gene regulation
Lactose metabolism only occurs when glucose concentrations low
• Catabolite activator protein (CAP) & cyclic AMP (cAMP)• CAP is activator
– Protein that binds to DNA and stimulates transcription of a gene
• cAMP concentration high when glucose concentration low• cAMP activates CAP
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Fig. 18.5
Negative and positive gene regulation
• CAP attaches to CAP-binding site on promoter
• Increases affinity of RNA polymerase for promoter
• Activators increase the rate of transcription
– As long as lactose is present, transcription can still occur
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Fig. 18.5