31 Gene regulation in bacteria
Lecture Outline 11/18/05
• Finish up from last time:
• Transposable elements
(“jumping genes”)
• Gene Regulation in Bacteria
– Transcriptional control
– Cells adjust to their environment by turning genes
on and off
• The operon concept
– Repressors, Inducers, Operators, Promoters
• Repressible operons (e.g. trp)
• Inducible operons (e.g. lac)
Transposable elements
• Normal and ubiquitous
– Prokaryotes• Genes transpose to/from cell’s
chromosome, plasmid, or a phage
chromosome.
– Eukaryotes• Genes transpose to/from same or a
different chromosome.
• Cause genetic changes
– Chromosome breaks
– Duplications
– Knock-out genes
I’ll talk about 2 kinds:
• Insertion sequences
• Ac/Ds elements in corn
• A third major class: Retrotransposons
– Uses RNA intermediate and reverse transcriptase
– Most Important class in mammalian genomes
Insertion sequence (IS) elements:
• Simplest type of transposable element
– Found in bacterial chromosomes and plasmids.
– Encode only genes for mobilization and insertion.
Inverted terminal
repeats
Integration of an Insertion Element
IS element carries
transposase gene
Transposase
recognizes terminal
repeats
Staggered cut at target site
Insert IS element
Fill in the gaps
Don’t worry
about the
details, just the
concept
Transposons
Have additional genes, such as those for antibiotic
resistance
• (examples Tn3 (ampicillin), Tn10 (tetracycline)
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
5
3
3
Inverted repeats
Figure 18.19b
Transposase gene
Barbara McClintock’s discovery of
transposons in corn:
•Kernel color alleles/traits
were “unstable”.
•McClintock concluded
transposon called “Ds”
inserted into the “C” gene
for colored kernels
Nobel prize, 1983
Transposon effects on corn kernel color.
Ac can make transposase
Ds can move, but lacks enzyme
Two transposable elements
in different sites
Ac activates
Ds
Normal gene for
purple kernels
Ds element
inserts into color
gene and
inactivates it
One method for
Conservative Transposition
“Cut and Paste”
Transposable element is cut out by
transposase and inserts in another
location.
No increase in the number of
transposable elements- just a
change in position
From Griffiths, Intro to Genetic Analysis
One method for replicative
transposition
From Griffiths, Intro to Genetic Analysis
Gene regulation in bacteria
E.coli bacteria eat
whatever we eat!
But ALL organisms must adjust to changes in
their environment and all have evolved
numerous control mechanisms.
Regulation of metabolism occurs at
two levels:
– Adjusting the activity of metabolic enzymes
already present
– Regulating the genes encoding the metabolic
enzymes
(a) Regulation of enzyme
activity
Precursor
Feedback
inhibition
(b) Regulation of enzyme
production
Enzyme 1 Gene 1
Enzyme 2 Gene 2
Regulation
of gene
expression
Enzyme 3 Gene 3
–
Enzyme 4 Gene 4
–
Enzyme 5 Gene 5
Tryptophan
Figure 18.20a, b
Types of Regulated Genes
• Constitutive genes are always expressed
– Tend to be vital for basic cell functions (often called
“housekeeping genes”)
• Inducible genes are normally off, but can be turned
on when substrate is present
• Common for catabolic enzymes (i.e. for the utilization of
particular resources)
• Repressible genes are normally on, but can be
turned off when the end product is abundant
• Common for anabolic enzymes
In bacteria, genes are often clustered
into operons
Operons have:
1. Several genes for metabolic enzymes
2. One promoter
3. An operator, or control site
(“on-off” switch)
4. A separate gene that makes a repressor or
activator protein that binds to the operator
P
R
P
O
1
2
3
The trp Operon
Controlled by a single
promoter and operator
5 genes: E, D, C, B, A
Same order as enzymes for trp synthesis
More Terminology
• Repressors and Activators are proteins that bind to
DNA and control transcription.
• Co-repressors and Inducers: small “effector”
molecules that bind to repressors or activators
The trp operon: regulated synthesis
of repressible enzymes
trp operon
Regulatory
gene
Promoter
Genes of operon
trpD
trpC
trpE
trpR
DNA
trpB
trpA
Operator
3
mRNA
RNA
polymerase
mRNA
5
5
E
D
C
B
A
Protein
Polypeptides that make up
enzymes for tryptophan synthesis
Figure 18.21a
Tryptophan absent -> repressor inactive -> operon “on”
Active repressor can
bind to operator and
block transcription
DNA
No RNA made
mRNA
Protein
Tryptophan
(corepressor)
Active
repressor
Tryptophan present -> repressor active -> operon “off”.
Figure 18.21b
Tryptophan changes the shape of
the repressor protein so it can bind
DNA
• The lac operon: regulated synthesis of
inducible enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
3
mRNA
Protein
No
RNA
made
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
lac operon
DNA
lacl
lacz
3
mRNA
5
lacA
RNA
polymerase
mRNA 5'
5
mRNA
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
Positive Gene Regulation
• Both the trp and lac operons involve negative
control of genes
– because the operons are switched off by the
active form of the repressor protein
• Some operons are also subject to positive
control
– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)
Positive Gene Regulation- CAP
– In E. coli, when glucose is always the preferred
food source
– When glucose is scarce, the lac operon is
activated by the binding of the catabolite activator
protein (CAP)
Promoter
DNA
lacl
lacZ
CAP-binding site
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
Active
and transcribe
CAP
Inactive lac
repressor
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.
If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces
Figure 18.23a
large amounts of mRNA for the lactose pathway.
• When glucose is abundant,
– CAP detaches from the lac operon, which
prevents RNA polymerase from binding to
the promoter
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA
polymerase
can’t bind
Inactive
CAP
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.
When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Figure 18.23b