To examine if binding of eEF1Bc to SL1 is important for stimulation of (-)-strand RNA synthesis by the viral RdRp, we performed an in vitro RNA synthesis assay using a mutated SL3-2-1 carrying the 'CUUG' tetraloop instead of the wt 'GAAA' tetraloop sequence ( Figure 4A ). Unlike for the wt SL3-2-1 RNA, eEF1Bc could not stimulate complementary RNA synthesis by the viral RdRp on the SL3-2-1cuug(+) template ( Figure 4A , lanes 7-10 versus 1-4). These data suggest that binding of eEF1Bc to the 'GAAA' tetraloop sequence of SL1 is important to stimulate (-)strand synthesis by the viral RdRp in vitro.