RNA Polymerase DNA-directed RNA polymerase catalyzes the synthesis of RNA on a DNA template. The core enzyme has three polypeptide subunits: alpha, beta, & beta' in a stoichiometry of α2ββ' & an additional subunit, omega.  Core RNA polymerase can bind to DNA and catalyze the synthesis of RNA, but has no specificity, it needs a 5th subunit sigma factor to make it the holoenzyme, with specificity.

α - required for assembly of the enzyme;
                 interacts with some regulatory proteins;
                 also involved in catalysis.

  beta- β - involved in catalysis; chain initiation and elongation.

  beta prime - β' - binds to the DNA template.

  sigma - σ - directs enzyme to promoter, begins specific transcription.

  omega - ω - required to restore denatured RNA polymerase
                     in vitro to its fully functional form

RNA polymerase
Image result for RNA
                polymerase core enzyme