Transcript The Operon 操縱元

The Operon 操縱元
a functioning unit of genomic
material containing a cluster of genes
under the control of a single
regulatory signal or promoter
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Ex Biochem c12-operon
12.1 Introduction
Figure 12.1
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12.2 Regulation Can Be
Negative or Positive
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Ex Biochem c12-operon
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In negative regulation,
a repressor protein
binds to an operator
to prevent a gene
from being expressed.
Figure 12.2
12.2 Regulation Can Be
Negative or Positive
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In positive regulation, a
transcription factor is
required to bind at the
promoter.
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This enables RNA
polymerase to initiate
transcription.
Enhancer, activator
Figure 12.3
Ex Biochem c12-operon
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Ex Biochem c12-operon
12.3 Structural Gene Clusters
Are Coordinately Controlled
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Genes coding for proteins that function in the same
pathway may be:
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Figure 12.4
located adjacent to one another
controlled as a single unit that is transcribed into a
polycistronic mRNA
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12.4 The lac Genes Are
Controlled by a Repressor
Ex Biochem c12-operon
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Transcription of the lacZYA gene cluster is controlled
by a repressor protein.
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The repressor binds to an operator that overlaps the
promoter at the start of the cluster.
The repressor protein is a tetramer of identical
subunits coded by the gene lacI.
Figure 12.5
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Ex Biochem c12-operon
12.5 The lac Operon Can Be Induced
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Small molecules that induce an operon are identical with or
related to the substrate for its enzymes.
β-galactosides are the substrates for the enzymes coded by
lacZYA.
In the absence of β-galactosides, the lac operon is expressed
only at a very low (basal) level.
Addition of specific β-galactosides induces 誘發
transcription of all three genes of the operon.
The lac mRNA is extremely unstable;
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as a result, induction can be rapidly reversed.
The same types of systems that allow substrates to induce
operons coding for metabolic enzymes can be used to allow
end-products to repress the operons that code for
biosynthetic enzymes.
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Ex Biochem c12-operon
Figure 12.06: lac expression responds to inducer.
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Ex Biochem c12-operon
12.6 Repressor Is Controlled
by a Small Molecule Inducer
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An inducer functions by converting the
repressor protein into a form with lower
operator affinity.
Repressor has two binding sites:
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one for the operator
one another for the inducer
Repressor is inactivated by an allosteric
interaction:
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Binding of inducer at its site changes the properties
of the DNA-binding site
Ex Biochem c12-operon
Inducer of lac Operon
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IPTG: common
inducer for lac
Operon used in lab
Similar structure to
lactose
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Although can NOT
be digested by betagalactosidase
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Ex Biochem c12-operon
Figure 12.07: A repressor
tetramer binds the operator to
prevent transcription.
Figure 12.08: Inducer inactivates
repressor allowing gene
expression.
http://www.youtube.com/watch?v=oBwtxdI1zvk
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Ex Biochem c12-operon
12.7 cis-Acting Constitutive
Mutations Identify the Operator
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Mutations in the operator cause
constitutive expression of all three
lac structural genes.
These mutations are cis-acting and
affect only those genes on the
contiguous 連續的 stretch of DNA.
Cis-acting
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Figure 12.9
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Referring to a regulatory sequence in
DNA (e.g., enhancer, promoter) that
can control a gene only on the same
chromosome.
In bacteria, cis-acting elements adjacent
or proximal to the genes they control,
whereas in eukaryotes they may also be
far away
Ex Biochem c12-operon
12.8 trans-Acting Mutations
Identify the Regulator Gene
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Mutations in the lacI gene:
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trans-acting
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are trans-acting
affect expression of all lacZYA clusters in the bacterium
Referring to DNA sequences encoding diffusible
proteins (e.g., transcription activators and repressors)
that control genes on different chromosomes
Mutations that eliminate lacI function:
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cause constitutive expression
are recessive
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Ex Biochem c12-operon
12.8 trans-Acting Mutations
Identify the Regulator Gene
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Mutations in the DNAbinding site of the repressor
are constitutive because the
repressor cannot bind the
operator.
Mutations in the inducerbinding site of the repressor:
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Figure 12.10
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prevent it from being
inactivated
cause uninducibility
Mutations in the promoter
are:
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uninducible
cis-acting
12.14 Repressor Protein
Binds to the Operator
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Figure 12.17
Ex Biochem c12-operon
Repressor protein binds to the double stranded
DNA sequence of the operator.
The operator is a palindromic sequence of 26 bp.
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Ex Biochem c12-operon
Operons in eukaryotes
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gene order in eukaryotes is NOT random.
numerous reports of gene clusters of related
function in eukaryotes, even humans
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significant tendency for genes from the same
metabolic pathway to cluster.
Extensive clustering of non-homologous
genes that are co-ordinately expressed in
eukaryotes, including humans
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Ex Biochem c12-operon
Operons in eukaryotes
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At the functional level, physical clustering
may be advantageous because it allows
groups of genes to be co-ordinately regulated
at the levels of nuclear organization and/or
chromatin.
The alleles could interact well by being colocalized in regions of chromosomes that
facilitate co-ordinate regulation
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Ex Biochem c12-operon
Hurst, 2004
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Ex Biochem c12-operon
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Osbourn, 2009