Genomic plus‐strand RNA synthesis by the brome mosaic virus (BMV) RNA replicase requires a sequence that is complementary to the binding site of the BMV helicase‐like protein

Summary Initiation of genomic plus‐strand RNA synthesis by the brome mosaic virus (BMV) replicase in vitro requires a 26‐nucleotide (nt) RNA sequence at the 3′ end of the minus‐strand RNA and a nontemplated nucleotide 3′ of the initiation cytidylate [ Sivakumaran, K. and Kao, C.C. (1999) J. Virol. 64 , 6415–6423]. At the 5′ end of this RNA is a 9‐nt sequence called the cB box, the complement of the previously defined B box. The cB box can not be functionally replaced by the B box and has specific positional and sequence requirements. The portion of the cB box that is required for RNA synthesis in vitro is well‐conserved in species in the Bromoviridae family. An equivalent RNA from Cucumber mosaic virus was unable to direct efficient RNA synthesis by the BMV replicase until the cB box was positioned at the same site relative to the BMV RNA and guanylates were present at positions +6 and +7 from the initiation cytidylate. These results further define the elements required for the recognition and initiation of viral genomic plus‐strand RNA synthesis and suggest that a sequence important for minus‐strand RNA synthesis is also required for plus‐strand RNA synthesis.