Question: 4. TRYPTOPHAN OPERON: A Repressible Operon with Attenuation The product of the regulatory gene R …

4. TRYPTOPHAN OPERON: A Repressible Operon with Attenuation

The product of the regulatory gene R is an inactive repressor
protein. The inactive repressor does not block transcription so the
products of the structural genes are produced and are able to
synthesize a constant supply of tryptophan for the cell. The
presence of tryptophan in the environment activates the repressor
which can then bind to the operator region and block transcription
of the structural genes.

The tryptophan operon contains an “attenuator” region in the
5’UTR sequence between the operator and the first structural gene.
The attenuator is transcribed by RNA polymerase, but if tryptophan
is present, the transcript forms a hairpin structure followed by a
string of uracils that terminates transcription. If tryptophan is
low, an alternative hairpin structure forms that allows
transcription to continue into the structural gene sequences.

A. Describe how this operon carries out transcription and
translation under the following conditions:

High tryptophan in the environment.

Low tryptophan in the environment together with rapid protein
synthesis in the cell (uses tryptophan up).

Rapid synthesis of tryptophan with a low rate of protein
synthesis in the cell (does not use tryptophan at a rapid
rate).

How many “start transcription” signals are there in this
operon?
How many “stop transcription” signals are there?

How many signals are there to start and stop translation?

   B. Consider events that could disrupt this operon
such as:

What mutations in the operator could make this operon
constitutive?
Could shut it off permanently?

What mutations in the promoter could alter the rate of
transcription?
How serious would each one probably be?

How would mutations in the regulator gene affect the operon?

How would mutations in the structural genes affect the
operon?

Which mutations would potentially have the greatest effect on
the ability of the bacterium to survive?

   C. Tryptophan levels in the cell are also
controlled by feedback inhibition, where tryptophan inhibits the
activity of the enzyme encoded by the trpE and trpD genes. When
tryptophan levels build up in the cell, which type of control would
be exerted first, feedback inhibition or repression of tryptophan
operon transcription? Be sure that you can explain your answer.