A structural model for fidelity in transcription.

Distances between the metal ions bound to the product terminus i site and the substrate i + 1 site of Escherichia coli RNA polymerase range from 5.0 to 5.6 A when the substrate is complementary to a template base and from 6.5 to 7.0 A for a noncomplementary relationship. The metal bound to the substrate at the i + 1 site exhibits a constant distance to the three phosphates on the substrate regardless of complementarity, but the distance to base and ribose protons changes. The differences in these geometric parameters are explained by the ability of the enzyme to assume two conformations, one to place correct nucleotide substrates in optimal position for bond formation and the other to prevent incorrect nucleotides from assuming such a position. In this scheme a metal-triphosphate complex can move toward or away from the terminal 3' OH group of the growing RNA chain, to assure fidelity of transcription.