A non-essential component of RNA polymerase (RNAP). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) RNAP is composed of a core of 2 alpha, a beta and a beta' subunit. The core is associated with a delta subunit, and at least one of epsilon or omega. When a sigma factor is associated with the core the holoenzyme is formed, which can initiate transcription. Belongs to the RNA polymerase subunit epsilon family.