The Arabinose Operon
The three structural genes (araB
) encode for enzymes needed for the
metabolism of the sugar arabinose in bacterial cells. AraB
, and araD
for the enzymes kinase, isomerase, and epimerase. Isomerase converts arabinose to ribulose. Kinase
converts ribulose to ribulose-5-phosphate. Then, epimerase converts ribulose-5-phosphate to
xyulose-5-phosphate, which enters the glycolytic pathway for carbon metabolism.
The arabinose operon also contains the arabinose C gene, which synthesizes itself.
gene regulates the expression of the structural genes and the araC
gene product. Thus, the araC
gene is auto regulated. When the level of araC
gene product is high in the cell, the araC
gene binds to its operator, preventing further
synthesis of the araC
The presence of both arabinose and the araC
gene product activates the expression of the BAD genes. Arabinose binds to the araC
product, causing a change in the shape
This new shape helps RNA polymerase to
bind to the BAD promoter, which sends a signal to begin the transcription of the structural genes
gene acts as a positive control for the BAD genes and a negative
control for its own synthesis.
Mutations at the araA
genes block the pathway of arabinose metabolism.
Bacterial cells can survive with the presence of arabinose, but can not utilize it. If there
is a mutation at the araD
gene, the cells will not survive due to the build up of L-ribulose-5P,
which is toxic to the cell. Mutation of the araC
gene prevents the synthesis of the araC
gene product. As a result, the structural genes are not expressed.