RNA polymerase (RNAP), also known as DNA-dependent RNA polymerase, is a large and complex enzyme of several polypeptide chain subunits that catalyzes the synthesis of all cellular RNA specifically on intact DNA templates. A complete RNAP (holoenzyme) must contain two functional parts, a sigma factor (σ) and a core enzyme consisting of one beta prime subunit (β’), one beta subunit (β) and two alpha subunits (α), where the core enzyme catalyzes RNA synthesis initiated by the sigma factor at specific sites. Moreover, a growing list of factors of RNAP, including M, CAP, psi and rho, are required for the transcription reaction.