L-arabinose operon

The L-arabinose operon of the model bacterium Escherichia coli has been a focus for research in molecular biology for over 40 years, and has been investigated extensively at the genetic, biochemical, physiological, and biophysical levels. It is controlled by a dual positive and negative system. There are 3 structural genes: araB, araA, and araD. They encode the metabolic enzymes for breaking down the monosaccharide sugar arabinose into D-xylulose-5-phosphate, which is then metabolised via the pentose phosphate pathway. The initiator region, containing an operator site as well as a promoter, is called araI (the last letter of araI is an uppercase letter "i"). Near this site lies the araC gene, which encodes an activator protein. The AraC protein binds to initiator region araI.

Metabolism of arabinose in E. coli

Substrate	Protein(s)	Function	Reversible	Product
L-arabinose	AraA	isomerase	yes	L-ribulose
L-ribulose	AraB	ribulokinase	no	L-ribulose-phosphate
L-ribulose-phosphate	AraD	epimerase	yes	D-xylulose-phosphate

Activation

If arabinose is present, it builds a complex: AraC + arabinose
This complex is needed for RNA polymerase to bind to the promoter and transcribe the ara operon. Also for activation the binding of another structure to araI is needed: CRP (formerly known as CAP) + cyclic AMP
So the activation depends on the presence of arabinose and cAMP.

Repression

If arabinose is absent, the AraC protein assumes a different conformation. In this conformation the AraC protein binds to the araI DNA region as well as to the ara0 DNA region. This causes the DNA to form a loop, preventing transcription of the downstream ara operon elements.

Sequence of the Operon:

araC : araO : araI : araB : araA : araD

See also

• The lac operon is another operon in E. coli that has been a focus for research in gene expression.

References

"Regulation of the L-arabinose operon of Escherichia coli". Trends in Genetics 16 (12): 559–565. December 2000. doi:10.1016/S0168-9525(00)02153-3. PMID 11102706.