activator(n = noun.substance) - (biology) any agency bringing about activation; a molecule that increases the activity of an enzyme or a protein that increases the production of a gene product in DNA transcription;
is a kind of substance
has particulars: activating agent, accelerator, catalyst, sensitiser, sensitizer
Antonym: inhibitor
Derived form verb activate₂

   any agency bringing about activation; (Biol.) a molecule that increases the activity of an enzyme or a protein that increases the production of a gene product in DNA transcription. [WordNet 1.5]

plasminogen activator, tissue plasminogen activator