E. coli RNA polymerase is an enzyme that catalyzes the formation of phosphodiester bonds between nucleotides during transcription. It is composed of five subunits - two α subunits, one β subunit, one β' subunit, and one ω subunit. Together with a sigma factor, these subunits form the holoenzyme which is required for transcription initiation by binding to promoter sequences on DNA. The sigma factor recognizes specific promoter sequences and directs the holoenzyme to begin RNA synthesis. E. coli has multiple sigma factors that direct transcription of different genes depending on environmental conditions.