Final answer:
When an activator protein binds to the activator binding site on DNA, it causes a shape change in the DNA, allowing it to interact with proteins at the promoter site and initiate transcription.
Step-by-step explanation:
When an activator protein binds to the activator binding site on DNA, it causes a shape change in the DNA. This shape change allows the activator protein to interact with proteins at the promoter site, which is necessary for transcription to occur. The activator protein enhances the interaction between RNA polymerase and the promoter, leading to the initiation of transcription.