English: The core catalytic domains of DNA and RNA polymerases. a Double-psi beta-barrel (DPBB) polymerases. RNAP, multisubunit DNA-dependent RNA polymerase from Thermus thermophilus (PDB ID: 1iw7); eRdRP, eukaryotic RNA-dependent RNA polymerase from Neurospora crassa (PDB ID: 2j7n); PolD, DP2 subunit of family D DNA polymerase from Pyrococcus abyssi (PDB ID: 5ijl). b RRM-fold polymerases. RRM, RNA-recognition motif-containing RNA-binding domain of human nucleolysin TIAR (PDB ID: 2cqi); RdRP, RNA-dependent RNA polymerase of poliovirus type 1 (PDB ID: 1ra7); PolA, family A DNA polymerase from Thermus aquaticus (PDB ID: 1taq); ssRNAP, single-subunit DNA-dependent RNA polymerase of bacteriophage T7 (PDB ID: 1msw); RT, reverse transcriptase of Moloney murine leukemia virus (PDB ID: 1mml); PolB, family B DNA polymerase from Thermococcus gorgonarius (PDB ID: 1tgo). c PolC-like polymerases. Polβ, DNA polymerase β from Rattus norvegicus (PDB ID: 1bpb); PolC, family C DNA polymerase from Thermus aquaticus (PDB ID: 2hpi). In b and c, the major secondary structure elements forming the core palm domain are indicated with numbers (for β-strands) and capital letters (for α-helices). Dashed lines indicate regions where insertions into the core palm domains have occurred; these have been omitted for visualization purposes