The essential nature of Ded1p for host mRNA translation, however, makes characterization of Ded1p function as a host factor difficult. Therefore, the combined use of in vivo and in vitro approaches might be necessary to dissect the function of Ded1p in RNA virus replication as shown in this paper. By using a Ded1pdepleted yeast CFE in combination with recombinant Ded1p allowed us to define that the ATPase (helicase) activity of Ded1p is required for efficient TBSV (+)-strand synthesis (see below). Ded1p also increased the RdRp activity of the FHV replicase, suggesting that recruitment of DEAD-box helicases might also be useful for additional small RNA viruses (see below).