Biochemical and genetic characterization of dengue virus methyltransferase

We report that dengue virus (DENV) methyltransferase sequentially methylates the guanine N-7 and ribose 2'-O positions of viral RNA cap (GpppA→m⁷GpppA→m⁷GpppAm). The order of two methylations is determined by the preference of 2'-O methylation for substrate m⁷GpppA-RNA to GpppA-RNA, and the 2'-O methylation is not absolutely dependent on the prior N-7 methylation. A mutation that completely abolished the 2'-O methylation attenuated DENV replication in cell culture, whereas another mutation that abolished both methylations was lethal for viral replication, suggesting that N-7 methylation is more important than 2'-O methylation in viral replication. The latter mutant with lethal replication could be rescued by trans complementation using a wild-type DENV replicon. Furthermore, we found that chimeric DENVs containing the West Nile virus methyltransferase, polymerase, or full-length NS5 were nonreplicative, but the replication defect could also be rescued through trans complementation using the wild-type DENV replicon.