Final answer:
The ara operon is subjected to positive regulation when arabinose is present, as the sugar binds to AraC protein, causing the complex to bind to the initiator site and promote transcription, unlike in negative regulation where a repressor inhibits transcription.
Step-by-step explanation:
The ara operon is under positive regulation when arabinose is present. This operon controls the production of enzymes for the metabolism of the sugar arabinose in bacteria. When arabinose is available, it binds to the AraC protein. The arabinose-AraC complex then binds to the initiator site and promotes transcription of the operon. This activation of transcription by the effect of arabinose exemplifies a clear case of positive regulation.
Contrary to negative regulation, such as seen with the lac operon in the presence of lactose, where the repressor is inactivated to allow transcription, in the ara operon scenario with arabinose, a protein (AraC) activates transcription. This activation is similar to the one effected by the CAP protein in the lac operon when glucose levels are low and lactose is present, promoting transcription. Therefore, to answer the question, when arabinose is present, the ara operon is subjected to positive regulation.