Functional Description
DNA-dependent RNA polymerases catalyze the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase I (Pol I) which synthesizes ribosomal RNA precursors. Besides, RNA polymerase I has intrinsic RNA cleavage activity. RPA190 and RPA135 both contribute to the polymerase catalytic activity and together form the Pol I active center. In addition, subunit RPA12 contributes a catalytic zinc ribbon that is required for RNA cleavage by Pol I. A single stranded DNA template strand of the promoter is positioned within the central active site cleft of Pol I. A bridging helix emanates from RPA190 and crosses the cleft near the catalytic site and is thought to promote translocation of Pol I by acting as a ratchet that moves the RNA-DNA hybrid through the active site by switching from straight to bent conformations at each step of nucleotide addition.
