Final answer:
The AraC protein can act as a transcriptional activator or inhibitor in the ara operon depending on arabinose availability. Without arabinose, AraC represses the operon; with arabinose, it binds to the inducer and activates transcription by promoting RNA polymerase binding to the promoter.
Step-by-step explanation:
The AraC protein serves as a crucial regulatory component in the expression of the ara operon in prokaryotic cells, specifically E. coli. This operon is responsible for the metabolism of the sugar arabinose. Depending on the presence or absence of arabinose, the AraC protein can either activate or repress transcription of the operon. When arabinose is not present, AraC acts as a repressor by binding to both the araO2 and araI1 sites, forming a DNA loop that inhibits RNA polymerase binding and thus preventing transcription. Conversely, when arabinose is available, it binds to AraC, causing a conformational change. This complex then binds to the initiator site araI2, serving as an activator by promoting the binding of RNA polymerase to the promoter, thereby initiating transcription.
In summary, AraC multi-functional behavior is due to its ability to change conformation upon binding with the inducer arabinose. The presence or absence of arabinose dictates whether AraC will repress or activate transcription of the ara operon by influencing the binding affinity of RNA polymerase to the promoter. This regulation reflects the cell's adaptive response to the availability of substrates, allowing for efficient utilization of energy sources.