Download Positive Control and Catabolite Repression

Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 16
Control of Gene Expression in
Prokaryotes
© 2014 W. H. Freeman and Company
16.1 The Regulation of Gene Expression
is Critical for All Organisms
• Genes and Regulatory Elements
• Levels of Gene Regulation
• DNA-Binding Proteins
Genes and Regulatory Elements
• Structural genes: encoding proteins
• Regulatory genes: encoding products that
interact with other sequences and affect the
transcription and translation of these sequences
• Regulatory elements: DNA sequences that are
not transcribed but play a role in regulating other
nucleotide sequences
Genes and Regulatory Elements
• Constitutive expression: continuously
expressed under normal cellular conditions
• Positive control: stimulate gene expression
• Negative control: inhibit gene expression
Levels of Gene Regulation
DNA-Binding Proteins
• Domains: ~ 60- 90 amino acids, responsible for
binding to DNA, forming hydrogen bonds with
DNA
• Motif: within the binding domain, a simple
structure that fits into the major groove of the
DNA
‒ Distinctive types of DNA-binding proteins based on
the motif
Concept Check 1
How do amino acids in DNA-binding proteins
interact with DNA?
a. By forming covalent bonds with DNA base
b. By forming hydrogen bonds with DNA base
c. By forming covalent bonds with sugars
Concept Check 1
How do amino acids in DNA-binding proteins
interact with DNA?
a. By forming covalent bonds with DNA base
b. By forming hydrogen bonds with DNA base
c. By forming covalent bonds with sugars
16.2 Operons Control Transcription in
Bacterial Cells
• Operon Structure
• Negative and Positive Control: Inducible and Repressible
Operons
• The lac Operon of E. coli
• lac Mutations
• Positive Control and Catabolite Repression
• The trp Operon of E. coli
Operon Structure
• Operon: promoter + additional sequences that
control transcription (operator) + structure
genes
• Regulator gene: DNA sequence encoding
products that affect the operon function, but are
not part of the operon
Concept Check 2
What is the difference between a structural gene
and a regulator gene?
a. Structural genes are transcribed into mRNA, but
regulator genes are not.
b. Structural genes have complex structures;
regulator genes have simple structure.
c. Structural genes encode proteins that function in
the structure of the cell; regulator genes carry out
metabolic reactions.
d. Structural genes encode proteins; regulator genes
control the transcription of structural genes.
Concept Check 2
What is the difference between a structural gene
and a regulator gene?
a. Structural genes are transcribed into mRNA, but
regulator genes are not.
b. Structural genes have complex structures;
regulator genes have simple structure.
c. Structural genes encode proteins that function in
the structure of the cell; regulator genes carry out
metabolic reactions.
d. Structural genes encode proteins; regulator genes
control the transcription of structural genes.
Negative and Positive Control: Inducible
and Repressible Operons
• Inducible operons: Transcription is usually off
and needs to be turned on.
• Repressible operons: Transcription is normally
on and needs to be turned off.
Negative and Positive Control: Inducible
and Repressible Operons
• Negative inducible operons: The control at the
operator site is negative. Molecule binding is to
the operator, inhibiting transcription. Such
operons are usually off and need to be turned
on, so the transcription is inducible.
• Inducer: small molecule that turns on the
transcription.
Negative and Positive Control: Inducible
and Repressible Operons
• Negative repressible operons: The control at
the operator site is negative. But such
transcription is usually on and needs to be
turned off, so the transcription is repressible.
• Corepressor: a small molecule that binds to the
repressor and makes it capable of binding to the
operator to turn off transcription.
Negative and Positive Control: Inducible
and Repressible Operons
• Positive inducible operon
• Positive repressible operon
Concept Check 3
In a negative repressible operon, the regulator
protein is synthesized as
.
a.
b.
c.
d.
an active activator
an inactive activator
an active repressor
an inactive repressor
Concept Check 3
In a negative repressible operon, the regulator
protein is synthesized as
.
a.
b.
c.
d.
an active activator
an inactive activator
an active repressor
an inactive repressor
The lac Operon of E. coli
•
•
•
•
A negative inducible operon
Lactose metabolism
Regulation of the lac operon
Inducer: allolactose
–lacI: repressor encoding gene
–lacP: operon promoter
–lacO: operon operator
The lac Operon of E. coli
• Structural genes
‒ lacZ: encoding β-galactosidases
‒ lacY: encoding permease
‒ lacA: encoding transacetylase
• The repression of the lac operon never
completely shuts down transcription.
Concept Check 4
In the presence of allolactose, the lac
repressor
.
a.
b.
c.
d.
binds to the operator
binds to the promoter
cannot bind to the operator
binds to the regulator gene
Concept Check 4
In the presence of allolactose, the lac
repressor
.
a.
b.
c.
d.
binds to the operator
binds to the promoter
cannot bind to the operator
binds to the regulator gene
lac Mutations
• Partial diploid: full bacterial chromosome + an
extra piece of DNA on F plasmid
• Structural gene mutations: affect the structure
of the enzymes, but not the regulations of their
synthesis
• lacZ+lacY−/lacZ−lacY+ produce fully functional
β-galactosidase and permease.
lac Mutations
• Regulator gene mutations: lacI− leads to
constitutive transcription of three structure
genes.
‒ lacI+ is dominant over lacI− and is trans acting.
A single copy of lacI+ brings about normal
regulation of lac operon.
‒ lacI+lacZ−/lacI−lacZ+ produce fully functional βgalactosidase.
lac Mutations
• Operator mutations: lacOc: C = constitutive
• lacOc is dominant over lacO+, which is cis
acting.
• lacI+lacO+Z−/lacI+lacOclacZ+ produce fully
functional β-galactosidase constitutively.
lac Mutations
• Promoter mutations
• lacP−: cis acting
• lacI+lacP−lacZ+/lacI+lacP+lacZ− fails to produce
functional β-galactosidase.
Positive Control and Catabolite Repression
• Catabolite repression: using glucose when
available, and repressing the metabolite of other
sugars.
• This is a positive control mechanism. The
positive effect is activated by catabolite activator
protein (CAP). cAMP is binded to CAP, together
CAP–cAMP complex binds to a site slightly
upstream from the lac gene promoter.
Positive Control and Catabolite Repression
• cAMP: adenosine-3′, 5′-cyclic monophosphate
• The concentration of cAMP is inversely
proportional to the level of available glucose.
Concept Check 5
What is the effect of high levels of glucose
on the lac operon?
a.
b.
c.
d.
Transcription is stimulated.
Little transcription takes place.
Transcription is not affected.
Transcription may be stimulated or inhibited,
depending on the levels of lactose.
Concept Check 5
What is the effect of high levels of glucose
on the lac operon?
a.
b.
c.
d.
Transcription is stimulated.
Little transcription takes place.
Transcription is not affected.
Transcription may be stimulated or inhibited,
depending on the levels of lactose.
The trp Operon of E. coli
• A negative repressible operon
• Five structural genes
• trpE, trpD, trpC, trpB, and trpA—five enzymes
together convert chorismate to typtophane.
Concept Check 6
In the trp operon, what happens to the trp
repressor in the absence of tryptophan?
a. It binds to the operator and represses
transcription.
b. It cannot bind to the operator and transcription
takes place.
c. It binds to the regulator gene and represses
transcription.
d. It cannot bind to the regulator gene and
transcription takes place.
Concept Check 6
In the trp operon, what happens to the trp
repressor in the absence of tryptophan?
a. It binds to the operator and represses
transcription.
b. It cannot bind to the operator and transcription
takes place.
c. It binds to the regulator gene and represses
transcription.
d. It cannot bind to the regulator gene and
transcription takes place.
16.3 Some Operons Regulate Transcription
Through Attenuation, the Premature
Termination of Transcription
• Attenuation: affects the continuation of
transcription, not its initiation. This action
terminates the transcription before it reaches the
structural genes.
• Attenuator
• Antiterminator
Attenuation in the trp Operon of E. coli
• Four regions of the long 5′ UTR (leader) region
of trpE mRNA
• When tryptophan is high, region 1 binds to
region 2, which leads to the binding of region
3 and region 4, terminating transcription
prematurely.
Attenuation in the trp Operon of E. coli
• Four regions of the long 5′ UTR (leader) region
of trpE mRNA
• When tryptophan is low, region 2 binds to
region 3, which prevents the binding of region
3 and region 4, and transcription continues.
16.4 RNA Molecules Control the
Expression of Some Bacterial Genes
• Antisense RNA: complementary to
targeted partial sequence of mRNA
• Riboswitches: molecules influence the
formation of secondary structures in
mRNA
• Ribozymes: mRNA molecules with
catalytic activity