A sigma factor is the promoter specificity subunit of eubacterial-type multisubunit RNA polymerases, those whose core subunit composition is often described as alpha(2)-beta-beta-prime. (This type of multisubunit RNA polymerase complex is known to be found in eubacteria and plant plastids). Although sigma does not bind DNA on its own, when combined with the core to form the holoenzyme, this binds specifically to promoter sequences, with the sigma factor making sequence specific contacts with the promoter elements. The sigma subunit is released from the elongating form of the polymerase and is thus free to act catalytically for multiple RNA polymerase core enzymes.