Component of the Mediator complex, a coactivator involved in regulated gene transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene- specific regulatory proteins to the basal RNA polymerase II transcription machinery. Mediator is recruited to promoters by direct interactions with regulatory proteins and serves as a scaffold for the assembly of a functional pre-initiation complex with RNA polymerase II and the general transcription factors. Phosphorylates the CTD (C- terminal domain) of the large subunit of RNA polymerase II (RNAp II), which may inhibit the formation of a transcription initiation complex. Phosphorylates CCNH leading to down-regulation of the TFIIH complex and transcriptional repression (PubMed:10993082). Recruited through interaction with MAML1 to hyperphosphorylate the intracellular domain of NOTCH, leading to its degradation (PubMed:15546612). Selectively regulates the interferon response by phosphorylating the transcription factor STAT1 transactivation domain (By similarity). {ECO:0000250|UniProtKB:Q8R3L8, ECO:0000269|PubMed:10993082, ECO:0000269|PubMed:15546612, ECO:0000269|PubMed:30905399}. This is the function of CDK8 (cyclin dependent kinase 8, Ensembl gene identifier ENSG00000132964).