Download Repressible Operons - MrsPalffysAPBio2013

Operon Vocabulary
‘self regulatory biological system in which the
• Feedback output (or response) affects the input; can be
•
•
positive or negative’
oscillates between two different
Allosteric Protein shapes depending upon what’s
bonded to allosteric site
Promoter location upstream of gene where RNA
polymerase binds to begin
transcription
• Gene sequence of DNA that codes for proteins
when segment of DNA is transcribed
• Gene Expression and translated into a functional
protein
What is an Operon?
A. System within a prokaryotic cell which is involved
in controlling gene expression
B. Operons usually contain clusters of genes which
code for proteins (enzymes) that are all involved in
one metabolic process
C. Operons are turned ‘on’ or ‘off’ by a DNA
switch (segment of DNA) called an operator
•
•
If operator is open, then RNA polymerase can bind
If operator is blocked, then RNA polymerase cannot bind
D. Operon = promoter + operator + gene cluster
E. Every prokaryotic gene cluster controlled by operon
system; you need to understand function of these two
operon systems:
trp operon
lac operon
F. There are two main kinds of operon regulatory systems:
•repressible operons (ex. trp operon)
Normally, genes are ‘ON’ and presence of a
co-repressor TURNS OFF gene expression
•inducible operons (ex. lac operon)
Usually genes are “OFF” but the presence of
a particular substances (inducer) TURNS ON
expression of one or more genes
Graphic Organizer
Characteristic
or Feature
Definition
Relationship to
metabolism
Repressor
protein
produced as
Example
Type of Gene
Regulation
Repressible
Operon
Inducible
Operon
Repressible Operons
(anabolic metabolic pathway)
A. Utilizes repressor protein which binds to operator and
prevents RNA polymerase from attaching to promoter for
transcription
B. Repressor protein created by a regulatory gene (regulator)
which is independent of the operon with which it represses
C. Repressor protein is allosteric ... has two distinct shapes
D. In repressible operon, repressor protein produced by
regulator is in inactive state
E. What the heck does all this mean???
Repressible Operons
(anabolic metabolic pathway)
F. Take a look at the trp operon in action! (figure 18.20a, b )
trp operon picture: identify components of operon
trp operon (layer 1)
trp operon (layer 2)
Trp Operon Animation (not from your textbook)
G. Summarize NOW!! {as a flow chart?!}
H. Generally, repressible operons are involved in anabolic pathways
The creation of a product (co-repressor) activates the repressor
to turn operon OFF (block binding of RNA polymerase at
promoter)
Repressible Operons
(anabolic metabolic pathway)
Figure 18.3
Repressible Operons
(anabolic metabolic pathway)
Figure 18.3
Repressible Operons
(anabolic pathway)
Figure 18.3
Inducible Operons
(catabolic metabolic pathway)
A. Regulatory gene creates active repressor protein that binds to
operator and therefore prevents transcription of gene cluster
B. An inducer is required to ‘inactivate’ repressor and unblock
operator...
C. Inducers are usually small molecules that are involved in the
catabolic pathway
D. Lactose is inducer of lac operon. Lactose must be present in
order to express genes and create enzymes necessary to
convert lactose ---> glucose + galactose
Inducible Operons
(catabolic metabolic pathway)
E. Check it out: (figure 18.4)
Lac Operon (with out lactose)
Lac Operon (with lactose in cell)
Lac Operon Animation (not from textbook)
Another Lac Operon Animation
F. How is lac operon ‘turned off’? Summarize how
inducible operons work in your own words!
G. Generally, inducible operons only transcribe catabolic
enzymes when molecule involved in catabolic pathway
is present in the environment & bacterial cell!
Inducible Operons
(catabolic metabolic pathway: Figure 18.4)
Inducible Operons
(catabolic metabolic pathway: Figure 18.4)
The lac Operon - review
lactose
Glucose +
Galactose
E. coli living in an environment with lactose:
• Inducer (lactose) binds to repressor bound to operator
• Shape of repressor protein changes, and repressor
disengages from operator
• RNA polymerase is now free to bind to DNA and transcribe
lactose-catabolizing genes!
Operon Review
Operator not blocked, RNA
polymerase access promoter
Gene ON
INDUCIBLE OPERON
REPRESSIBLE
OPERON
(catabolic pathway)
(anabolic pathway)
Gene OFF
Active repressor bound to operator
blocking RNA polymerase
How are inducible and
repressible operons
SIMILAR?
• Explain your answer...
How are inducible and
repressible operons
DIFFERENT?
• Explain your answer...
Characteristic
or Feature
Definition
Relationship to
metabolism
Repressible
Operon
Inducible
Operon
Genes normally
‘ON’, but can be
turned off by corepressor –
Anabolism
Genes normally
‘OFF, but can be
turned on by inducer
build stg nec. for survival
Break  stg in envir.
Catabolism –
Repressor
protein
produced as
Inactive form
Active form
Example
Trp operon
lac operon
Type of Gene
Regulation
Negative: repressor
bound to operator
prevents expression
Negative: repressor
bound to operator
prevents expression
What is the selective advantage to regulating gene
expression in prokaryotic cells??
Operons allow bacteria to adapt quickly to changes in
their environment.
These single-celled organisms must adapt quickly
because they don’t have a lot of ATP to waste making
unnecessary proteins
The action of enzymes within the cell alter the raw
materials present in the environment in which the
bacteria lives
Quick adaptation = survival & >> reproduction!
“Volume control” of lac Operon
E.coli w/ lactose
& low [glucose]
• Genes for lactose catabolism
are ON
• B/c [glucose] is low, lots of
lactose catabolism is required
to generate glucose
• Therefore genes are ON and
LOUD:
– Make lots of mRNA
– Do lots of transcription of
genes
– Make many catabolic enzy.
E.coli w/ lactose & high
[glucose]
Genes for lactose
catabolism are ON
B/c [glucose] is high, it is
not as important to
metabolize lactose as
source for glucose
Therefore genes are ON,
but are QUIET
Make less mRNA
Do little transcription
Make few catabolic enz.
How does a bacterium
measure the concentration of
glucose in the cell?
How does a bacterium know to
increase the ‘volume’ of
transcription based on the
amount of glucose in the cell?
Positive Gene Regulation of lac Operon
• Recall that genes for lactose catabolism are
only expressed when lactose is present
• What if the cell is in an environment with an
adequate supply of glucose and lactose?
• Which molecule is the first choice of bacteria to
use to create ATP: lactose or glucose?
GLUCOSE
•How can the bacterium relate [glucose] to gene
expression?
Allosteric interaction between a
protein and a molecule called
cyclic AMP (cAMP)
cyclic AMP (cAMP)
adenylyl cyclase
ATP
-------------->
cAMP
•cAMP accumulates when glucose is scarce
•The greater the [cAMP], the more cAMP binds to
a protein called cAMP receptor protein (CRP).
•CRP is an activator of transcription!
cAMP and CRP
• cAMP binds to allosteric site of CRP and
changes shape of CRP
• The new shape of CRP allows CRP to
bind to a location upstream of the lac
promoter
• CRP bends DNA and somehow makes it
easier for RNA polymerase to bind to DNA
and begin transcription
• The binding of CRP to DNA facilitates
transcription and is therefore an example
of positive gene regulation!
Positive Gene Regulation (figure 18.5)
LACTOSE present & glucose scare so cAMP levels are
high; LOTS of lactose-catabolizing enzymes transcribed
(active CRP promotes binding of mRNA at promoter)
Positive Gene Regulation (figure 18.5)
Lactose present AND glucose present so cAMP
levels are low; few lactose-catabolizing
enzymes transcribed
Relate operon to recombinant plasmid
from transformation lab & biotechnology
Cut arabinose gene with restriction enzyme, but
promoter remains intact.
Insert jellyfish gene downstream from arabinose
promoter in plasmid...
Arabinose (in agar medium) binds to operator to
facilitate binding of RNA polymerase to promoter to
transcribe gene.
Now expression of jelly fish gene proceeds and
bacteria colonies GLOW GREEN!!
Is the arabinose operon inducible or repressible,
based on this information?