The major functions of the subunits are: The β' subunit contains part of the active center responsible for RNA synthesis. The β subunit contains the rest of the active center responsible for RNA synthesis. The α subunit is present in two copies per molecule of RNA Polymerase. The α subunit contains determinants for assembly of RNA Polymerase and for interactions with regulatory factors. The small ω subunit facilitates assembly of RNA Polymerase and stabilizes assembled RNA Polymerase. Sigma factors, upon associating with the core enzyme, increase specificity for its promoters and allow transcription to initiate at correct sites.