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Objective 3: TSWBAT recognize the processes by which bacteria respond to environmental changes by regulating transcription.
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Overview: Differential Expression of Genes
Prokaryotes and eukaryotes alter gene expression in response to their changing environment Multicellular eukaryotes also develop and maintain multiple cell types Gene expression is often regulated at the transcription stage, but control at other stages is important, too 2
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Bacteria often respond to environmental change by regulating transcription
Natural selection has favored bacteria that produce only the gene products needed by the cell A cell can regulate the production of enzymes by feedback inhibition or by gene regulation Gene expression in bacteria is controlled by a mechanism described as the operon model 3
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(a) Regulation of enzyme activity (b) Regulation of enzyme production
Figure 15.2 Precursor Feedback inhibition trpE gene Enzyme 1 Regulation of gene expression trpD gene Enzyme 2 trpC gene trpB gene Figure 15.2 Regulation of a metabolic pathway Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production 4
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Operons: The Basic Concept
A group of functionally related genes can be coordinately controlled by a single “on-off switch” The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control 5
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The operon can be switched off by a protein repressor
The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase The repressor is the product of a separate regulatory gene 6
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For example, E. coli can synthesize the amino acid tryptophan
The repressor can be in an active or inactive form, depending on the presence of other molecules A corepressor is a molecule that cooperates with a repressor protein to switch an operon off For example, E. coli can synthesize the amino acid tryptophan 7
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By default the trp operon is on and the genes for tryptophan synthesis are transcribed
When tryptophan is present, it binds to the trp repressor protein, which then turns the operon off The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high 8
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Polypeptide subunits that make up enzymes for tryptophan synthesis
Figure 15.3 trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Stop codon Start codon 3 mRNA 5 mRNA 5 E D C B A Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on DNA No RNA made Figure 15.3 The trp operon in E. coli: regulated synthesis of repressible enzymes mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off 9
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Polypeptide subunits that make up enzymes for tryptophan synthesis
Figure 15.3a DNA trp operon Regulatory gene Promoter Promoter Genes of operon trpR trpE trpD trpC trpB trpA Operator RNA polymerase Stop codon Start codon mRNA 3 mRNA 5 5 E D C B A Protein Figure 15.3a The trp operon in E. coli: regulated synthesis of repressible enzymes (part 1: tryptophan absent) Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on 10
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(b) Tryptophan present, repressor active, operon off
Figure 15.3b DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) Figure 15.3b The trp operon in E. coli: regulated synthesis of repressible enzymes (part 2: tryptophan present) (b) Tryptophan present, repressor active, operon off 11
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription The trp operon is a repressible operon An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription 12
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By itself, the lac repressor is active and switches the lac operon off
The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose By itself, the lac repressor is active and switches the lac operon off A molecule called an inducer inactivates the repressor to turn the lac operon on 13
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Enzymes of the lactose pathway are called inducible enzymes
For the lac operon, the inducer is allolactose, formed from lactose that enters the cell Enzymes of the lactose pathway are called inducible enzymes Analogously, the enzymes for tryptophan synthesis are said to be repressible enzymes 14
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Video: lac Repressor Model
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(a) Lactose absent, repressor active, operon off
Figure 15.4 Regulatory gene Promoter Operator DNA lacI IacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA IacI IacZ IacY IacA RNA polymerase Figure 15.4 The lac operon in E. coli: regulated synthesis of inducible enzymes 3 mRNA mRNA 5 5 Protein -Galactosidase Permease Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on 16
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(a) Lactose absent, repressor active, operon off
Figure 15.4a Regulatory gene Promoter Operator DNA lacI IacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Figure 15.4a The lac operon in E. coli: regulated synthesis of inducible enzymes (part 1: lactose absent) Protein (a) Lactose absent, repressor active, operon off 17
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Allolactose (inducer)
Figure 15.4b DNA lac operon IacI IacZ IacY IacA RNA polymerase mRNA 3 mRNA 5 5 Protein -Galactosidase Permease Transacetylase Inactive repressor Figure 15.4b The lac operon in E. coli: regulated synthesis of inducible enzymes (part 2: lactose present) Allolactose (inducer) (b) Lactose present, repressor inactive, operon on 18
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Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor 19
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Positive Gene Regulation
E. coli will preferentially use glucose when it is present in the environment When glucose is scarce, CAP (catabolite activator protein) acts as an activator of transcription CAP is activated by binding with cyclic AMP (cAMP) Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription 20
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When glucose levels increase, CAP detaches from the lac operon, and transcription proceeds at a very low rate, even if lactose is present CAP helps regulate other operons that encode enzymes used in catabolic pathways 21
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Inactive lac repressor Inactive CAP Allolactose
Figure 15.5 Promoter DNA lacI IacZ CAP-binding site RNA polymerase binds and transcribes Operator Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA lacI IacZ Figure 15.5 Positive control of the lac operon by catabolite activator protein (CAP) CAP-binding site Operator RNA polymerase less likely to bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized 22
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RNA polymerase binds and transcribes
Figure 15.5a Promoter DNA lacI IacZ CAP-binding site Operator RNA polymerase binds and transcribes Active CAP cAMP Inactive lac repressor Figure 15.5a Positive control of the lac operon by catabolite activator protein (CAP) (part 1: glucose scarce) Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized 23
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polymerase less likely to bind
Figure 15.5b Promoter DNA lacI IacZ CAP-binding site Operator RNA polymerase less likely to bind Inactive CAP Inactive lac repressor Figure 15.5b Positive control of the lac operon by catabolite activator protein (CAP) (part 2: glucose present) (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized 24
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