Control of template positioning during de novo initiation of RNA synthesis by the bovine viral diarrhea virus NS5B polymerase.

The RNA-dependent RNA polymerase of the hepatitis C virus and the bovine viral diarrhea virus(BVDV)is able to initiate RNA synthesis denovo in the absence of a primer. Previous crystallographic data have pointed to the existence of a GTP-specific binding site (G-site) that is located in the vicinity of the active site of the BVDV enzyme. Here we have studied the functional role of the G-site and present evidence to show that specific GTP binding affects the positioning of the template during de novo initiation. Following the formation of the first phosphodiester bond, the polymerase translocates relative to the newly synthesized dinucleotide, which brings the 5'-end of the primer into the G-site, releasing the previously bound GTP. At this stage, the 3'-end of the template can remain opposite to the 5'-end of the primer or be repositioned to its original location before RNA synthesis proceeds. We show that the template can freely move between the two locations, and both complexes can isomerize to equilibrium. These data suggest that the bound GTP can stabilize the interaction between the 3'-end of the template and the priming nucleotide, preventing the template to overshoot and extend beyond the active site during de novo initiation. The hepatitis C virus enzyme utilizes a dinucleotide primer exclusively from the blunt end; the existence of a functionally equivalent G-site is therefore uncertain. For the BVDV polymerase we showed that de novo initiation is severely compromised by the T320A mutant that likely affects hydrogen bonding between the G-site and the guanine base. Dinucleotide-primed reactions are not influenced by this mutation, which supports the notion that the G-site is located in close proximity but not at the active site of the enzyme.