RNA polymerase III contains seventeen subunits in yeasts and in human cells. Twelve of these are akin to RNA polymerase I or II and the other five are RNA pol III-specific, and form the functionally distinct groups (i) Rpc31-Rpc34-Rpc82, and (ii) Rpc37-Rpc53. Rpc31, Rpc34 and Rpc82 form a cluster of enzyme-specific subunits that contribute to transcription initiation in S.cerevisiae and H.sapiens. There is evidence that these subunits are anchored at or near the N-terminal Zn-fold of Rpc1, itself prolonged by a highly conserved but RNA polymerase III-specific domain .
DNA-directed RNA polymerases EC (also known as DNA-dependent RNA polymerases) are responsible for the polymerisation of ribonucleotides into a sequence complementary to the template DNA. In eukaryotes, there are three different forms of DNA-directed RNA polymerases transcribing different sets of genes. Most RNA polymerases are multimeric enzymes and are composed of a variable number of subunits. The core RNA polymerase complex consists of five subunits (two alpha, one beta, one beta-prime and one omega) and is sufficient for transcription elongation and termination but is unable to initiate transcription. Transcription initiation from promoter elements requires a sixth, dissociable subunit called a sigma factor, which reversibly associates with the core RNA polymerase complex to form a holoenzyme [PUBMED:3052291]. The core RNA polymerase complex forms a