DNA-dependent RNA polymerases catalyze the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Common component of RNA polymerases I (Pol I) and III (Pol III) which synthesize ribosomal RNA precursors and small RNAs, such as 5S rRNA and tRNAs, respectively. RPC40 is part of the polymerase core and may function as a clamp element that moves to open and close the cleft (PubMed:18160037, PubMed:24153182, PubMed:24153184). Plays an important role in targeting retrotransposons Ty integration upstream of pol III-transcribed genes such as tRNA genes, allowing Ty1, Ty2 and Ty4 to proliferate and yet minimizing genetic damage (PubMed:25931562). Component of the RNA polymerase I (Pol I) complex consisting of 14 subunits: RPA135, RPA190, RPC40, RPA14, RPB5, RPO26, RPA43, RPB8, RPA12, RPB10, RPC19, RPC10, RPA49 and RPA34 (PubMed:8516295, PubMed:11717393, PubMed:12407181, PubMed:18160037, PubMed:24153184, PubMed:24153182). The complex is composed of a horseshoe-shaped core containing ten subunits (RPA135, RPA190, RPB5, RPO26, RPB8, RPB10, RPC10, RPA12, RPC19 and RPC40) where RPA135 and RPA190 form the DNA-binding cleft. Outside of the core, RPA14 and RPA43 form the stalk that mediates interactions with transcription initiation factors and newly synthesized RNA. Component of the RNA polymerase III (Pol III) complex consisting of at least 17 subunits (PubMed:8516295, PubMed:10384303, PubMed:10393904). Interacts with the RPC19/RPAC2 (PubMed:8516295) and RPC53/RPC4 (PubMed:10393904). Interacts with retrotransposons Ty integrase, targeting Ty1, Ty2 and Ty4 integration upstream of pol III-transcribed genes (PubMed:25931562). Present with 13000 molecules/cell in log phase SD medium. Belongs to the archaeal Rpo3/eukaryotic RPB3 RNA polymerase subunit family.