coactivator

noun

co·​ac·​ti·​va·​tor (ˈ)kō-ˈak-tə-ˌvā-tər 

: a protein or protein complex that activates genetic transcription usually by binding to a transcription factor

Gene-regulatory proteins that do not themselves bind DNA but assemble on DNA-bound gene regulatory proteins are often termed coactivators or corepressors, depending on their effect on transcription initiation.

—Bruce Alberts et al., Molecular Biology of the Cell