Final answer:
The protein complex that mediates the interaction between RNA polymerase II and regulatory transcription factors is called the Mediator.
Step-by-step explanation:
The protein complex that mediates the interaction between RNA polymerase II and regulatory transcription factors is called the Mediator. In eukaryotes, the Mediator complex plays a crucial role in the recruitment and activation of RNA polymerase II, facilitating the initiation of transcription. It acts as a bridge between the transcription factors and RNA polymerase II, helping to bring them together at the promoter region of the gene.