Assembly, Purification, and Pre-steady-state Kinetic Analysis of Active RNA-dependent RNA Polymerase Elongation Complex*

Abstract

Background: Previous studies have failed to reconstitute an active replication complex with hepatitis C virus (HCV) RNA-dependent RNA polymerase.

Results: The replication complex from HCV was assembled, purified, and characterized.

Conclusion: A highly active replication complex can be formed with HCV polymerase that catalyzes fast and processive RNA replication.

Significance: A purified and active replication complex is essential for mechanistic studies and drug discovery.

Abstract

NS5B is the RNA-dependent RNA polymerase responsible for replicating hepatitis C virus (HCV) genomic RNA. Despite more than a decade of work, the formation of a highly active NS5B polymerase·RNA complex suitable for mechanistic and structural studies has remained elusive. Here, we report that through a novel way of optimizing initiation conditions, we were able to generate a productive NS5B·primer·template elongation complex stalled after formation of a 9-nucleotide primer. In contrast to previous reports of very low proportions of active NS5B, we observed that under optimized conditions up to 65% of NS5B could be converted into active elongation complexes. The elongation complex was extremely stable, allowing purification away from excess nucleotide and abortive initiation products so that the purified complex was suitable for pre-steady-state kinetic analyses of polymerase activity. Single turnover kinetic studies showed that CTP is incorporated with apparent K _d and k _pol values of 39 ± 3 μ m and 16 ± 1 s ⁻¹, respectively, giving a specificity constant of k _pol/ K _d of 0.41 μ m ⁻¹ s ⁻¹. The kinetics of multiple nucleotide incorporation during processive elongation also were determined. This work establishes a novel way to generate a highly active elongation complex of the medically important NS5B polymerase for structural and functional studies.