text: Viral/prokaryotic RNA-directed RNA polymerases, along with many single-subunit DNA-directed polymerases, employ a fold whose organization has been linked to the shape of a right hand with three subdomains termed fingers, palm, and thumb. Only the palm subdomain, composed of a four-stranded antiparallel beta sheet with two alpha helices, is well conserved among all of these enzymes. In  RdRp, the palm subdomain comprises three well-conserved motifs (A, B, and C). Motif A (D-x(4,5)-D) and motif C (GDD) are spatially juxtaposed; the aspartic acid residues of these motifs are implied in the binding of Mg and/or Mn. The asparagine residue of motif B is involved in selection of ribonucleoside triphosphates over dNTPs and, thus, determines whether RNA rather than DNA is synthesized. The domain organization and the 3D structure of the catalytic centre of a wide range of  RdRps, even those with a low overall sequence homology, are conserved. The catalytic center is formed by several motifs containing a number of conserved amino acid residues.
  Eukaryotic RNA interference requires a cellular RNA-dependent RNA polymerase (c RdRp). Unlike the "hand" polymerases, they resemble simplified multi-subunit DNA-dependent RNA polymerases (DdRPs), specifically in the catalytic β/β subunits, in that they use two sets of double-psi β-barrels in the active site. QDE1 () in Neurospora crassa', which has both barrels in the same chain, is an example of such a c RdRp enzyme. Bacteriophage homologs of c RdRp, including the similarly single-chain DdRp yonO (), appear to be closer to c RdRps than DdRPs are.