Practice question

Catalysis in RNA polymerase active center depends on two divalent magnesium ions coordinated by invariant aspartate triad DFDGD motif in β' subunit and additional aspartates in β subunit. Metal A lowers pKa of RNA 3' hydroxyl activating it for nucleophilic attack on alpha-phosphate of incoming NTP, while Metal B neutralizes developing negative charge and facilitates pyrophosphate leaving group departure. Chelators like EDTA stripping Mg2+ abolish polymerization. Substituting calcium fails to support chemistry due to geometry differences. Thus Mg2+ is universal essential cofactor for both bacterial and eukaryotic RNA polymerases implementing two-metal ion mechanism.