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G en e Regulation Regulation of a Metabolic Pathway Precursor Feedback inhibition trpE gene An abundance of tryptophan can inhibit the activity of the first enzyme in the pathway (feedback inhibition) Enzyme 1 trpD gene Regulation of gene expression Enzyme 2 trpC gene trpB gene OR... Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production An abundance of tryptophan can repress the expression of the genes encoding all subunits of the enzymes in the pathway (more long term response) The trp Operon •There are 5 enzymes in the tryptophan Precursor Feedback inhibition pathway. They are grouped together (COORDINATELY CONTROLLED) in a transcription unit where a single promoter serves all 5 genes for enzyme production. •A single “on/off” switch, called an operator, is located in the promoter. Enzyme 1 Enzyme 2 trp operon Promoter Promoter Genes of operon DNA trpR Regulatory gene trpE Operator Start codon 3 Regulation mRNA RNA mRNA of gene 5 5 polymerase expression Protein E Inactive repressor trpD trpC trpB trpA B A Stop codon D C Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on The figure above is an example of an OPERON. An operon is a unit of genetic function in bacteria and phages, consisting of a promoter, operator, and coordinately related gene cluster whose products function in a common way. The trp Operon Precursor •Normally trp operon is turned on. Feedback inhibition A TRP REPRESSOR PROTEIN can bind to the operator and block attachment of RNA polymerase, repressing the transcription of the genes for tryptophan synthesis. Enzyme 1 Enzyme 2 trp operon Promoter Promoter Genes of operon DNA trpR Regulatory gene trpE Operator Start codon 3 Regulation mRNA RNA mRNA of gene 5 5 polymerase expression Protein E Inactive repressor trpD trpC trpB trpA B A Stop codon D C Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on The trp Operon Precursor •The trp repressor protein is allosteric. Feedback inhibition DNA It has 2 shapes - active and inactive. •When tryptophan is present it binds to the repressor protein, activating it. The repressor then binds to the operator blocking RNA polymerase. •The trp operon is a REPRESSIBLE OPERON because it is usually on, but can be repressed when tryptophan binds allosterically with the regulatory protein. •Tryptophan is a COREPRESSOR because it cooperates with repressor proteins to switch the operon off. Enzyme 2 No RNA made mRNA Active repressor Protein Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off DNA No RNA made mRNA Active repressor Protein Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off Lac Operon Precursor •INDUCIBLE OPERONS are usually off, but can be turned on. A molecule, the INDUCER, inactivates the repressor and initiates transcription. •The LAC OPERON is an example of an inducible operon. Feedback inhibition Enzyme 2 Regulatory gene Promoter Operator lacI DNA No RNA made 3 mRNA 5 lac operon lacZ DNA 3 mRNA 5 RNA polymerase Active repressor (a) Lactose absent, repressor active, operon off lacY -Galactosidase Permease Allolactose (inducer) lacA RNA polymerase mRNA 5 Protein Protein lacZ lacI Inactive repressor (b) Lactose present, repressor inactive, operon on Transacetylase Lac Operon •A regulatory gene codes for an allosteric repressor protein that can switch off Precursor Feedback inhibition the lac operon by binding to the promoter. •Unlike the trp repressor, the lac repressor is active by itself, binding to the operator and switching the lac operon off. •A specific molecule (the INDUCER) inactivates the repressor. Enzyme 2 Regulatory gene Promoter Operator lacI DNA No RNA made 3 mRNA 5 lac operon lacZ DNA 3 mRNA 5 RNA polymerase Active repressor (a) Lactose absent, repressor active, operon off lacY -Galactosidase Permease Allolactose (inducer) lacA RNA polymerase mRNA 5 Protein Protein lacZ lacI Inactive repressor (b) Lactose present, repressor inactive, operon on Transacetylase Lac Operon Precursor Feedback inhibition •Without the bound repressor, the lac operon is transcribed for lactose utilizing enzymes. Enzyme 2 Regulatory gene Promoter Operator lacI DNA No RNA made 3 mRNA 5 lac operon lacZ DNA 3 mRNA 5 RNA polymerase Active repressor (a) Lactose absent, repressor active, operon off lacY -Galactosidase Permease Allolactose (inducer) lacA RNA polymerase mRNA 5 Protein Protein lacZ lacI Inactive repressor (b) Lactose present, repressor inactive, operon on Transacetylase Anabolic Pathways Precursor Feedback inhibition •Repressible operons are usually anabolic pathways. By suspending production of an end product when it is already present in sufficient quantity, the cell can save resources. Enzyme 2 Negative Control Precursor •Inducible operons are usually catabolic pathways. •By producing appropriate enzymes only when the nutrient is Feedback inhibition available, the cell avoids wasting energy and precursors making proteins that are not needed. Enzyme 2 Repressible and inducible operons are two examples of NEGATIVE CONTROL/NEGATIVE GENE REGULATION. Operons are switched off by active forms of the repressor proteins. Positive Gene Regulation Precursor Feedback inhibition • Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription • When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP Promoter Operator DNA lacI lacZ Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose Enzyme 2 • Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription RNA polymerase binds and transcribes CAP-binding site (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA lacI CAP-binding site Inactive CAP Operator lacZ RNA polymerase less likely to bind Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized Positive Gene Regulation Precursor • When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate Feedback inhibition • CAP helps regulate other operons that encode enzymes used in catabolic pathways Promoter Operator DNA lacI lacZ RNA polymerase binds and transcribes CAP-binding site Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose Enzyme 2 (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA lacI CAP-binding site Inactive CAP Operator lacZ RNA polymerase less likely to bind Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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