Structural homology among different polymerases

 There is a structural homology among DNA dependent RNA polymerases, reverse transcriptases and RNA dependent RNA polymerases. All polymerases have a common RNA polymerase domain that resembles to human hand, which consists of fingers, palm and thumb. The combination of alpha helix and beta sheets of protein composes this formation, and the central area of palm domain contains the most important amino acids, GDN or GDD. Many studies regarding the mutagenesis of this sequence have revealed that these amino acids play an important role for nucleotide selection during polymerization of de novo synthesized RNA.