1. A type of transcription factor involved in assembling proteins to form an initiation complex at the promoter of a gene in readiness for transcription. Activators can bind to an enhancer site some distance upstream of the coding region and also to a promoter element near the start site, causing the DNA to form a loop. Some activators also direct remodelling of chromatin by catalysing the acetylation of histones required for unwinding of DNA. This provides access to other transcription factors as a prelude to transcription. Compare repressor.

2. A substance that—by binding to an allosteric site on an enzyme—enables the active site of the enzyme to bind to the substrate.

3. Any compound that potentiates the activity of a drug or other foreign substance in the body.