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