Some of the information below can be some help in regards to our lab #2. Arabinose is an aldopentose (a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group.) Arabinose can be obtained from plant polysaccharides such as gums and hemicelluloses. One specific type of arabinose is the L-arabinose operon of the model bacterium Escherichia coli. It 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: 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 as well to araI as to ara0 region. Now the DNA is forming a loop. This prevents transcription of the ara operon.
Sequence of the Operon:
araC : araO : araI : araB : araA : araD
*Colin Schroeder
