Question: 3. The Arabinose Operon: Inducible Operon under both Positive and Negative Control For the arabin…

3. The Arabinose Operon: Inducible Operon under both Positive
and Negative Control

For the arabinose operon, the same protein, the product of the
araC gene, exerts both negative and positive control. The araC
product is a negative regulator (active repressor) when arabinose
is not bound to it. AraC binds to both araI and araO creating a
looped DNA structure that prevents RNA polymerase from attaching to
the promoter. AraC is a positive regulator (enhancing
transcription) when it binds to arabinose. The arabinose + araC
product complex binds to the araI region containing the promoter
for the araB, araA and araD structural genes to stimulate
transcription.

This operon is also under the positive control of the cAMP + CAP
complex. Transcription of the structural genes is optimized when
both the arabinose + araC and cAMP + CAP complexes are attached to
their binding sites on the common promoter for araB, araA and araD.
Transcription of the structural genes is rare in the absence of
these positive regulators.

A. Show how the operon works under the following conditions:

High arabinose and high glucose in the environment.

High arabinose and low glucose in the environment.

Low arabinose and high glucose in the environment.

Low arabinose and low glucose in the environment.

B. Consider the effects of mutations on each part of this
system: (in all cases, assume the environment is high in arabinose
and low in glucose)

deletion of the I region.

deletion of the O region.

mutation in the araC gene such that the product cannot bind to
arabinose.

mutation in the araC gene such that the product can bind to
arabinose but the araC + arabinose complex cannot bind to the I
region.

mutation in the CAP gene such that the product cannot bind to
cAMP.

mutation of the I region so that it can no longer bind the
arabinose + ara C complex.