A large protein complex which acts as a signaling adaptor protein that allows communication between the various regulatory and functional components of GENETIC TRANSCRIPTION including dna polymerase ii; GENERAL transcription factors; and transcription factors that are bound to upstream ENHANCER elements. The mediator complex was originally studied in YEAST where at least 21 subunits were identified. Many of the yeast subunits are homologs to proteins in higher organisms that are found associated with specific nuclear receptors such as THYROID HORMONE RECEPTORS and vitamin d RECEPTORS.