1. 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

2. Ex Biochem c12-operon
12.1 Introduction
Figure 12.1

3. 12.2 Regulation Can Be Negative or Positive
Ex Biochem c12-operon
12.2 Regulation Can Be Negative or Positive
In negative regulation, a repressor protein binds to an operator to prevent a gene from being expressed.
Figure 12.2

4. 12.2 Regulation Can Be Negative or Positive
Ex Biochem c12-operon
12.2 Regulation Can Be Negative or Positive
In positive regulation, a transcription factor is required to bind at the promoter.
This enables RNA polymerase to initiate transcription.
Enhancer, activator
Figure 12.3

5. 12.3 Structural Gene Clusters Are Coordinately Controlled
Ex Biochem c12-operon
12.3 Structural Gene Clusters Are Coordinately Controlled
Genes coding for proteins that function in the same pathway may be:
located adjacent to one another
controlled as a single unit that is transcribed into a polycistronic mRNA
Figure 12.4

6. 12.4 The lac Genes Are Controlled by a Repressor
Ex Biochem c12-operon
12.4 The lac Genes Are Controlled by a Repressor
Transcription of the lacZYA gene cluster is controlled by a repressor protein.
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

7. 12.5 The lac Operon Can Be Induced
Ex Biochem c12-operon
12.5 The lac Operon Can Be Induced
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;
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.

8. Figure 12.06: lac expression responds to inducer.
Ex Biochem c12-operon
Figure 12.06: lac expression responds to inducer.

9. 12.6 Repressor Is Controlled by a Small Molecule Inducer
Ex Biochem c12-operon
12.6 Repressor Is Controlled by a Small Molecule Inducer
An inducer functions by converting the repressor protein into a form with lower operator affinity.
Repressor has two binding sites:
one for the operator
one another for the inducer
Repressor is inactivated by an allosteric interaction:
Binding of inducer at its site changes the properties of the DNA-binding site

10. Inducer of lac Operon IPTG: common inducer for lac Operon used in lab
Ex Biochem c12-operon
Inducer of lac Operon
IPTG: common inducer for lac Operon used in lab
Similar structure to lactose
Although can NOT be digested by beta-galactosidase

11. Figure 12.08: Inducer inactivates repressor allowing gene expression.
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.

12. 12.7 cis-Acting Constitutive Mutations Identify the Operator
Ex Biochem c12-operon
12.7 cis-Acting Constitutive Mutations Identify the Operator
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
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
Figure 12.9

13. 12.8 trans-Acting Mutations Identify the Regulator Gene
Ex Biochem c12-operon
12.8 trans-Acting Mutations Identify the Regulator Gene
Mutations in the lacI gene:
are trans-acting
affect expression of all lacZYA clusters in the bacterium
trans-acting
Referring to DNA sequences encoding diffusible proteins (e.g., transcription activators and repressors) that control genes on different chromosomes
Mutations that eliminate lacI function:
cause constitutive expression
are recessive

14. 12.8 trans-Acting Mutations Identify the Regulator Gene
Ex Biochem c12-operon
Mutations in the DNA-binding site of the repressor are constitutive because the repressor cannot bind the operator.
Mutations in the inducer-binding site of the repressor:
prevent it from being inactivated
cause uninducibility
Mutations in the promoter are:
uninducible
cis-acting
Figure 12.10

15. 12.14 Repressor Protein Binds to the Operator
Ex Biochem c12-operon
12.14 Repressor Protein Binds to the Operator
Repressor protein binds to the double stranded DNA sequence of the operator.
The operator is a palindromic sequence of 26 bp.
Figure 12.17

16. Operons in eukaryotes gene order in eukaryotes is NOT random.
Ex Biochem c12-operon
Operons in eukaryotes
gene order in eukaryotes is NOT random.
numerous reports of gene clusters of related function in eukaryotes, even humans
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

17. Ex Biochem c12-operon
Operons in eukaryotes
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 co-localized in regions of chromosomes that facilitate co-ordinate regulation

18. Ex Biochem c12-operon
Hurst, 2004

19. Ex Biochem c12-operon
Osbourn, 2009