trp Operon Recall Cells respond to their environments by modifying gene expression. Prokaryotic cells can individually adapt to their changing environments by regulating the expression of their genes. Bacteria can adjust to their environments using a special sequence of DNA known as an operon, which controls the synthesis of protein in response to the environment. An operon has three key areas on the DNA strand: the promoter, the operator, and the transcription unit. The bacteria Escherichia coli need the amino acid tryptophan to survive. Amazingly, if tryptophan is not readily available in its environment, E. coli will actually make its own tryptophan from another compound by activating a metabolic pathway called the trp operon (trp for tryptophan). This metabolic process “shuts off’ as soon as tryptophan is readily available again. How the trp Operon Works Tryptophan is made using a sequence of five different enzymatic reactions. The genes that code for these five enzymes are clustered together on the same chromosome. A single promoter transcribes the entire cluster at once. In this way, the cell can turn “off” or “on” the entire series of functionally associated genes simultaneously. Let’s look at the molecules that cause these switches to be “on” or “off”. 1. How does gene regulation involving the trp repressor protein differ from what you observed with the lac repressor in the previous lesson? (2 points) 2. Based on your understanding of gene regulation in the cell and the function of the trp operon, describe how this process would be affected if there was a mutation in the operator region so that the operator could not carry out its function. (4 points) 3. What would be the result in this process of a genetic mutation that altered the shape of the trp repressor protein? (4 points)

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Unit Assignment: trp Operon
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trp Operon
Recall
Cells respond to their environments by modifying gene expression.
Prokaryotic cells can individually adapt to their changing environments by
regulating the expression of their genes.
Bacteria can adjust to their environments using a special sequence of DNA
known as an operon, which controls the synthesis of protein in response to
the environment.
An operon has three key areas on the DNA strand: the promoter, the operator,
and the transcription unit.
The bacteria Escherichia coli need the amino acid tryptophan to survive. Amazingly,
if tryptophan is not readily available in its environment, E. coli will actually make its
own tryptophan from another compound by activating a metabolic pathway called
the trp operon (trp for tryptophan). This metabolic process “shuts off’ as soon as
tryptophan is readily available again.

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How the trp Operon Works
Tryptophan is made using a sequence of five different enzymatic reactions.
The genes that code for these five enzymes are clustered together on the
same chromosome. A single promoter transcribes the entire cluster at once. In
this way, the cell can turn “off” or “on” the entire series of functionally
associated genes simultaneously. Let’s look at the molecules that cause these
switches to be “on” or “off”.

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When Tryptophan is Absent
The switch for transcribing tryptophan is automatically always “on” until the
levels of tryptophan in the cell’s environment build up enough to cause active
trp repressor proteins switch it “off”. trp repressor molecules are regulatory
proteins that are constantly being made at a low rate by the cell.
The trp repressor molecule by itself doesn’t inhibit the operator, but high
tryptophan levels increase the likelihood that tryptophan molecules will bind
to the repressor’s allosteric site. In this way, tryptophan itself is acting as a
corepressor, because the trp repressor needs to be bound to the tryptophan
in order to have any effect on the operator.

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When Tryptophan is Present
When the tryptophan binds to the trp repressor, the trp repressor becomes attracted to
the operator and binds to it. This blocks the ability of the promoter (aka RNA
polymerase) to attach to the operator site in order to transcribe the genetic sequence.
Because the promoter cannot transcribe the sequence, the cell stops producing
tryptophan. This type of feedback system, where the transcription is always “on” until it
is “stopped” (or “repressed” by a repressor protein), is called a repressible operon.

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1. How does gene regulation involving
the trp repressor protein differ from
what you observed with the lac
repressor in the previous lesson? (2
points)
The trp repressor protein is different because the RNA
polymerase occurs when tryptophan is present. Tryptophan
binds with the repressor and activates it. The activated
repressor binds to the operator and blocks transcription. In the
lac repressor protein when lactose is present lactose binds to
the repressor causing it to not be able to bind to the operator,
allowing transcription to occur. When the trp repressor is
activated it stops transcription while when the lac repressor is
activated it allows transcription to occur.

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2. Based on your understanding of gene
regulation in the cell and the function
of the trp operon, describe how this
process would be affected if there was a
mutation in the operator region so that
the operator could not carry out its
function. (4 points)

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3. What would be the result in
this process of a genetic mutation
that altered the shape of the trp
repressor protein? (4 points)