L-arabinose operon
The L-arabinose operon, also called the ara or araBAD operon, is an operon that encodes enzymes needed for the catabolism of arabinose in Escherichia coli. It has both positive and negative regulation and is activated allosterically.
It has been a focus for research in molecular biology since 1970, and has been investigated extensively at its genetic, biochemical, physiological, and biophysical levels.
In E. coli, arabinose is converted to xylulose 5-phosphate, an intermediate of the pentose phosphate pathway.
The structural genes, which encode enzymes for arabinose catabolism, are araB, araA, and araD (collectively known as araBAD). The regulator gene is araC. The genes araBAD and araC are transcribed in opposite directions.
D-xylulose 5-phosphate and D-ribulose-5-phosphate are metabolites in the pentose phosphate pathway.
The operators are araI and araO2. The operators lie between the AraC.
AraI lies between the structural genes and the operator. The araI1 and araI2 are DNA-binding sites that, when occupied by AraC, induce expression.
(Distances not to scale)
The ara operon is regulated by the AraC protein. If arabinose is absent, the dimer AraC protein represses the structural gene by binding to araI1 and araO2 and the DNA forms a loop. The loop prevents RNA polymerase from binding to the promoter of the ara operon, thereby blocking transcription.
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