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 f rom 5.0 to 5.6 Angstrom when the substrate is complementary to a templ ate base and from 6.5 to 7.0 Angstrom for a noncomplementary relations hip. The metal bound to the substrate at the i + 1 site exhibits a con stant 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 abi lity of the enzyme to assume two conformations, one to place correct n ucleotide substrates in optimal position for bond formation and the ot her 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,