The Arabinose Operon
The arabinose operon is found in E. coli. It is a group of functionally related genes that work together to breakdown arabinose when it is present. The arabinose operon is composed of:
Structural genes: AraA, AraB, AraD each codes for a different enzyme that aids in the breaking down of arabinose.
Regulatory gene: AraC – a gene that codes for a regulatory protein which regulates the expression of itself and the expression of the structural genes.
Two operator sites: AraO1, AraO2. These are the sites where the regulatory protein binds if the repression of a gene is necessary.
Inducer site: AraI- In this site, if the regulatory protein is bound to it alone, it helps activate the expression of structural genes. However, if the regulatory gene is bound to it and the AraO2 simultaneously, then expression of structural genes is repressed.
CRP binding site- this site helps indirectly in the expression of structural genes.
Two promoter sites: Pc and Pbad- These promoter sites are where the RNA polymerase binds to in order for the transcription of regulatory and structural genes to occur.
In the absence of arabinose, the structural genes are not needed to be expressed. As a result, AraC binds to AraI and AraO2 simultaneously forming a loop in the DNA. This loop prevents the binding of RNA polymerase to Pbad and thus the structural genes are not transcribed.
In the presence of arabinose, the arabinose binds to AraC making it bind to AraI only (Positive control) and removing in it from the AraO2 (Negative control). Therefore, RNA polymerase has access to Pbad and transcription of the structural genes occurs. Also, the CRP binding to CRP site assists the AraC to bind only to AraI.
In addition, AraC also auto regulates the expression of its own gene product. When the concentration of AraC is sufficient, the AraC gene product binds AraO1. This causes it to repress the RNA polymerase from binding to Pc site and transcription of AraC does not occur. While the AraC bound to this region also assists in the transcription of structural genes, if the concentration of AraC is too low, the AraC doesn’t bind to AraO1 and therefore the transcription of AraC continues.
After the transcription, the three enzymes necessary for the digestion of arabinose are ready. AraA codes for Arabinose Isomerase which converts arabinose to ribulose. Then Ribolokinase which is coded by AraB phosphorylates ribulose. Finally, Rubulose-5-phosphate epimerase encoded by AraD converts ribulose-5-phosphate to xylose-5-phosphate, which is later metabolized by the glycolytic pathway.
Here is a summary of the steps takes to break down arabinose:
L-ribose Isomerase L-ribulose kinase L-ribulose-5P epimerase