Arabinose is a five carbon sugar that can be used by E.coli as an alternative carbon source. The enzymes necessary for the metabolism of arabinose are coded for by the arabinose operon. The arabinose operon has a complex regulatory system. It was studied and explained by a scientist, Ellis Englesberg soon after the Jacob and Monod described the lac operon. He came to the conclusion that the arabinose operon can be regulated both positively and negatively in a similar manner as the lactose operon. Therefore the arabinose operon is also an inducible operon. In E.coli cells growing in the absence of arabinose, the three different enzymes involved in its metabolism are present in the cell in very small amounts and there is no expression of the operon. This is an adaptive mechanism that ensures that these enzymes needed to catabolize arabinose are only produced in sufficient amounts when arabinose is present in the environment. The arabinose operon also exhibits catabolite repression. A cAMP-CRP complex must be formed in order for the positive expression of the arabinose operon to occur. High levels of glucose in the environment will repress the arabinose operon due to low levels of the cAMP molecule. This is similar to the conditions necessary for lactose to be utilized as a carbon source. The arabinose operon will only express its genes if arabinose is the best carbon source present in the environment.