Slide 1
Differential Expression of GenesProkaryotes and eukaryotes
precisely regulate gene expression in response to environmental
conditionsIn multicellular eukaryotes, gene expression regulates
development and is responsible for differences in cell typesRNA
molecules play many roles in regulating gene expression in
eukaryotes 2014 Pearson Education, Inc.1Concept 18.1: Bacteria
often respond to environmental change by regulating
transcriptionNatural selection has favored bacteria that produce
only the products needed by that cellA cell can regulate the
production of enzymes by feedback inhibition or by gene
regulationOne mechanism for control of gene expression in bacteria
is the operon model 2014 Pearson Education, Inc.Operons: The Basic
ConceptA cluster of functionally related genes can be coordinately
controlled by a single on-off switchThe switch is a segment of DNA
called an operator usually positioned within the promoterAn operon
is the entire stretch of DNA that includes the operator, the
promoter, and the genes that they control 2014 Pearson Education,
Inc.3 The operon can be switched off by a protein repressorThe
repressor prevents gene transcription by binding to the operator
and blocking RNA polymeraseThe repressor is the product of a
separate regulatory gene 2014 Pearson Education, Inc.4 The
repressor can be in an active or inactive form, depending on the
presence of other moleculesA corepressor is a molecule that
cooperates with a repressor protein to switch an operon offFor
example, E. coli can synthesize the amino acid tryptophan when it
has insufficient tryptophan 2014 Pearson Education, Inc.5 By
default the trp operon is on and the genes for tryptophan synthesis
are transcribedWhen tryptophan is present, it binds to the trp
repressor protein, which turns the operon off The repressor works
only in the presence of its corepressor tryptophan; thus the trp
operon is turned off (repressed) if tryptophan levels are high 2014
Pearson Education, Inc.6
Figure 18.3aPromoterDNAtrpRRegulatory
geneRNApolymerasemRNA53ProteinInactiverepressormRNA 5(a) Tryptophan
absent, repressor inactive, operon onPromotertrp operonGenes of
operontrpEtrpDtrpCtrpBtrpAOperatorStart codonPolypeptide subunits
that make upenzymes for tryptophan synthesisEDCB A 2014 Pearson
Education, Inc.7Figure 18.3a The trp operon in E. coli: regulated
synthesis of repressible enzymes (part 1: tryptophan absent)
Figure
18.3bDNAmRNAProteinActiverepressortrpRtrpETryptophan(corepressor)(b)
Tryptophan present, repressor active, operon off35NoRNAmade 2014
Pearson Education, Inc.8Figure 18.3b The trp operon in E. coli:
regulated synthesis of repressible enzymes (part 2: tryptophan
present)Repressible and Inducible Operons:Two Types of Negative
Gene RegulationA repressible operon is one that is usually on;
binding of a repressor to the operator shuts off transcriptionThe
trp operon is a repressible operonAn inducible operon is one that
is usually off;a molecule called an inducer inactivates the
repressor and turns on transcription 2014 Pearson Education, Inc.
The lac operon is an inducible operon and contains genes that code
for enzymes used in the hydrolysis and metabolism of lactoseBy
itself, the lac repressor is active and switches the lac operon
offA molecule called an inducer inactivates the repressor to turn
the lac operon on 2014 Pearson Education, Inc.10
Figure 18.4aPromoterDNARegulatory
genemRNA53OperatorRNApolymeraseActiverepressorNo RNAmadeIacZ(a)
Lactose absent, repressor active, operon offProteinlacI 2014
Pearson Education, Inc.11Figure 18.4a The lac operon in E. coli:
regulated synthesis of inducible enzymes (part 1: lactose
absent)
Figure 18.4blacZlacYlacA(b) Lactose present, repressor inactive,
operon on5DNARNA
polymerasemRNA3ProteinInactiverepressorAllolactose(inducer)mRNA
5
lacIStart codonPermeaseTransacetylase-Galactosidaselac operon
2014 Pearson Education, Inc.12Figure 18.4b The lac operon in E.
coli: regulated synthesis of inducible enzymes (part 2: lactose
present)Video: Cartoon Rendering of the lac Repressor from E.
coli
2014 Pearson Education, Inc. Inducible enzymes usually function
in catabolic pathways; their synthesis is induced by achemical
signalRepressible enzymes usually function in anabolic pathways;
their synthesis is repressed by high levels of the end
productRegulation of the trp and lac operons involves negative
control of genes because operons are switched off by the active
form of the repressor 2014 Pearson Education, Inc.14Positive Gene
RegulationSome operons are also subject to positive control through
a stimulatory protein, such as catabolite activator protein (CAP),
an activator of transcriptionWhen glucose (a preferred food source
of E. coli) is scarce, 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 2014 Pearson Education, Inc.15 When glucose levels
increase, CAP detaches from the lac operon, and transcription
returns to a normal rateCAP helps regulate other operons that
encode enzymes used in catabolic pathways 2014 Pearson Education,
Inc.16
Figure 18.5aDNAPromoterOperatorCAP-binding
sitecAMPActiveCAPInactiveCAPRNApolymerasebinds and
transcribeslacIAllolactoseInactive lacrepressor(a) Lactose present,
glucose scarce (cAMP level high):abundant lac mRNA synthesizedlacZ
2014 Pearson Education, Inc.17Figure 18.5a Positive control of the
lac operon by catabolite activator protein (CAP) (part 1: glucose
scarce)
Figure 18.5bPromoterDNAlacZCAP-binding siteRNApolymerase
lesslikely to bindOperatorInactiveCAPInactive lacrepressor(b)
Lactose present, glucose present (cAMP level low):little lac mRNA
synthesizedlacI 2014 Pearson Education, Inc.18Figure 18.5b Positive
control of the lac operon by catabolite activator protein (CAP)
(part 2: glucose present)Operons that can be turned on or off are
inducible or repressableOperons that are always on are called
constitutive 2014 Pearson Education, Inc.On unless specifically
turned off = negative control example = repressor action in the lac
operonOff unless specifically turned on = positive control example
= CAP protein in lac operon
2014 Pearson Education, Inc.
