Final answer:
The tail of the Mediator complex, denoted as number 3, is the component often utilized by transactivators to recruit Mediator to the promoter during transcription initiation.
Step-by-step explanation:
The component of the Mediator that is often used by transactivators to recruit the Mediator to the promoter corresponds to number 3, the tail of the Mediator. Mediator is a multi-protein complex that functions as a transcriptional co-activator in all eukaryotes. Specifically, transactivators interact with the tail domain of Mediator to enhance transcription. The head and middle complexes of Mediator also play significant roles in the function of this complex, but it is the tail that is typically involved with interactions with transactivators and thus, the recruitment to the promoter.
During the process of transcription initiation, various transcription factors and other components like TFIID and TFIIH bind to the core promoter region to facilitate the initiation of transcription by RNA polymerase II. Promoter-proximal elements, such as the TATA box, CAAT box, and GC box, are critical for the efficient and precise initiation of transcription. Enhancers are DNA elements that can promote the transcription of genes, often by facilitating the interaction between transcription factors bound at distal control elements and the core promoter via proteins like Mediator.