How is RNA polymerase activated?

RNA polymerase is activated by promoters. These are DNA sequences that are required to turn a gene on or off. The transcription process is initiated when RNA polymerase locates and binds to the promoter sequence, which is located near the beginning of a gene. The promoter essentially guides the RNA polymerase enzyme to the correct region on the DNA to begin transcribing.  

RNA polymerase II is different from the other RNAP enzymes in that it is activated by acetyl coenzyme A (acetyl-CoA).