Download 19. Gene Regulation

Biology 212 General Genetics
Lecture 19: Gene Regulation
Spring 2007
Reading: Chap. 9 pp. 318-326
Lecture Outline:
1. Transcriptional control
2. Lactose operon: negative control
3. Lactose mutants
4. Positive control (optional)
Lecture:
1. Transcriptional control
Gene regulation: Control of when, where and under what conditions genes are
transcribed and translated.
Transcriptional regulation


Transcribe gene when product is needed, turn off transcription when product is
not needed.
Efficient way of controlling gene expression—don’t have to commit to processes
of transcription and translation.
Operon: Group of genes that are coordinately regulated in bacteria. Usually, a single
polycistronic mRNA is produced from which multiple proteins are translated.
2. Lactose operon



E. coli prefers to use glucose as a carbon source.
Can use lactose as a carbon source, if necessary.
Need to induce (switch on) gene expression.
Lactose metabolism
Lactose is a disaccharide (made of two sugar subunits)
Lactose --> glucose + galactose
Needed for lactose metabolism:

β-galactosidase: Enzyme that breaks down lactose into glucose and galactose

Permease: Protein that transports lactose into cell

Transacetylase: Enzyme, exact function unknown
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Components of the lactose operon
Structural genes: Genes that code for proteins



Lac Z: gene for β-galactosidase
Lac Y: gene for permease
Lac A: gene for transacetylase
Regulatory genes and elements: Control expression of the structural genes



Lac I: gene for repressor protein; shuts off the operon
Lac O: operator=element on DNA that repressor binds
Lac P: promoter=site where transcription begins
Inducer: Chemical signalling molecule

Form of lactose (allolactose)
Organization of the lac operon Fig. 9.4
I
P
O
Z
Y
A
When lactose is absent, glucose is present --> Operon is OFF
1)
2)
3)
4)
I gene is transcribed and translated to give repressor protein
Repressor binds to operator
Transcription is prevented
Operon is off (repressed)
When lactose (inducer) is present, glucose is low or absent --> Operon is ON
1)
2)
3)
4)
5)
6)
I gene is transcribed and translated to produce repressor
Repressor binds inducer (form of lactose)
Conformation of repressor is altered
Repressor can’t bind operator
RNA polymerase can bind promoter and transcribe genes in the operon (ZYA)
The polycistronic mRNA is translated to give β-galactosidase, permease and
transacetylase proteins.
7) Lactose is transported into the cell and utilized as a carbon source.
3. Lactose mutants

E. coli strains with mutations in the lactose metabolic genes were used by Jacob
and Monod in the early 1960’s to elucidate the lac operon.
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
Partial diploid strains, produced when an F factor integrates into the chromosome
(form Hfr), but is excised imprecisely, taking some bacterial genes with it.
Mutants in structural genes:
Lac ZLac ALac Y-
No protein or defective protein produced
Mutants in regulatory elements and genes: Interfere with normal regulation of the
genes, not the genes themselves
Lac I- = lac Ic
No repressor protein, constituitive expression of lactose operon
genes
Operon is always ON; no inducer required
Lac Oc
Operator constituitive mutant; can’t bind repressor
Operon is always ON; no inducer required
Lac P-
Promoter mutant; no transcription of lactose operon genes
Operon always OFF
How do these mutations affect lactose operon expression singly and in combination?
Table from Problem 9.16
Levels of activity of lactose metabolic enzymes
Genotype
I+O+Z+Y+
I+OcZ+Y+
Uninduced
Z
0.1
25
Uninduced
Y
0.1
25
Induced
Z
100
100
Induced
Y
100
100
Genotypes of partial diploids
Problem 9.16 part (b)
F’ I+P+O+Z-Y-/I-O+Z+Y+
What do these symbols mean?
F’ = F factor containing some bacterial genes
Second genotype =bacterial chromosome
How is F’ formed?
F factor integrates into bacterial chromosome (forms Hfr); when it comes out, sometimes
it takes some of the adjacent bacterial genes with it, creating an F’.
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What would be the phenotype of this cell?




I+ is produced in cell from F’.
I+ binds to operator on the chromosome (repressor is diffusable and can act in
trans)
In the absence of inducer, the operon is OFF.
When inducer (lactose) is added, repressor binds to the inducer and is unable to
bind the operator. The operon is ON.
Phenotype is wild type.
4. Positive control (may not get to in lecture)
To be fully on, the lactose operon also needs a positive regulator.

The positive regulator is highly sensitive to glucose levels in the environment.

Glucose has the effect of repressing the lac operon even when lactose is present.

If glucose is present --> cAMP concentration is low
CRP = cyclic AMP receptor protein: binds to cAMP
 Complex binds to the lac operon at a site near the promoter (see Fig. 9.7)
 Enhances binding of RNA polymerase to the promoter.
High glucose
Low glucose
low cAMP
high cAMP
low transcription
more transcription
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