Gene Machine-lac operon guide

Bacterial Gene Regulation: The Lac Operon…
Key terms: Operon,
promoter, polycistronic
mRNA, operator.
Bacteria have an efficient genome organization strategy that, when combined
with signal transduction pathways, allow them to respond rapidly, correctly,
Key concept: bacteria
and appropriately to their environment. First, bacterial genes are organized
respond correctly and
into operons. An operon is a set of genes with related functions that are all
appropriately to their
regulated by one promoter. If the promoter is activated by a transcription
environment by
factor, the bacterial RNA polymerase will make a single messenger RNA that
regulating sets of genes
contains the transcript of all the genes in the operon. This messenger RNA is
called operons.
called polycistronic mRNA, it codes for multiple separate polypeptides. This
is efficient because the cell can activate all the genes needed to appropriately
respond to the environmental signal at once. It also means that the bacteria
will express all the proteins in the operon, or none. Study the figure below to understand the
organization of the lac operon and note the location of the, regulatory gene, promoter, and
structural genes (lacZ, lacY, and lacA).
LAC OPERON
Regulatory
Gene
Promoter
Structural Genes
Polycistronic mRNA produced
Another important feature of the operon system is that they contain a
stretch of DNA called the operator. The operator is usually located in between
the promoter and the structural genes in the operon. The operator controls
access to the structural genes. The regulatory gene encodes a regulatory
protein called the repressor. In its native state, the repressor is active and binds
to the operator DNA, blocking RNA polymerase from transcribing the
structural genes in the operon. Study the structure of the lac operon below and
note how the repressor blocks transcription of the structural genes.
Key terms: operator,
regulatory gene,
repressor.
Key concept: The
regulatory gene
produces a repressor
that binds to the
operator in the absence
of lactose, preventing
transcription of the lac
operon.
Why do bacteria make a repressor protein to prevent the expression of the lac
operon structural genes? To understand the logic of this system you need to
know what the function of the lac operon structural genes are. The lac operon
structural genes code for enzymes necessary to use lactose as an energy
source. E. coli are aerobic bacteria that can use glucose or lactose as an
energy source, but prefer to use glucose because it is easier to breakdown.
Also, it takes energy to make the enzymes needed to break down lactose. E.
coli will not waste valuable energy and resources making the enzymes needed
for lactose catabolism unless there is not enough glucose available, and of
course, lactose is present in the environment.
Key concept: organisms
need to respond
correctly and
appropriately to their
environment. It is
energy efficient to
repress the production
of unnecessary proteins.
How does the presence of lactose allow the transcription of lac operon
Key concept: lactose
structural genes? If lactose is in the environment of the cell, some will enter
inactivates the
repressor, so it cannot
and bind to the repressor protein. The binding of lactose to the repressor
bind to the operator.
changes the shape of the repressor and prevents it from binding to the
operator. Essentially, lactose inactivates the repressor. Now RNA polymerase
can transcribe the lac operon genes and make the enzymes needed to break down lactose.
What type of feedback system is this? Biological systems have two types of regulation, negative
or positive. In a negative feedback system, the stimulus is opposite to the response. In positive
feedback, the stimulus is in the same direction as the response. Consider the diagram below of
how the lac operon responds to stimulus:
Stimulus
Response
Lactose present (+)
Repressor inactivated (-)
Lactose absent (-)
Repressor activated (+)
Clearly the stimulus is opposite the response, so the lac operon is considered
to be regulated by negative feedback.
You can visualize and manipulate this system using the phet.colorado.edu
simulation: Gene Machine: the lac operon.
Key concept: the lac
operon is an example of
negative feedback
(stimulus is opposite the
response).
Play with the simulation and make sure you understand how it works. Use the simulation to test
and determine the outcome of the following mutation scenarios (assume the mutation disrupts
the function of the DNA). To simulate the mutation, remove the specified portion of the operon
from the DNA and determine the outcome. Be sure to state the direct consequence, and then
whether or not the cell will be able to break down lactose. Be sure to clear out all the remaining
enzymes before you test a new scenario.
Scenario
Direct Consequence
Can the cell break down
lactose? If so, in what
situations?
Scenario 1: no mutations are
present.
Scenario 2: The lacI promoter
is mutated
Scenario 3: the lacI gene is
mutated
Scenario 4: the lac Z promoter
is mutated
Scenario 5: the operator is
mutated
Scenario 6: the lac Z gene is
mutated
1. In what situations would the cell always have enzymes necessary to break down lactose,
regardless of lactose being present?
2. In what situations would the cell never able to break down lactose present in the cell?
3. Ms. Taylor remembers how the lac operon is regulated with the phrase:
“Repress the Repressor.” Explain why this is a good way to remember the regulation of the
lac operon?

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