A model describing the functions of Ded1p and GAPDH during tombusvirus replication.
The (−)repRNA, which is shown with a “closed” 3′ end due to either a secondary structure within the promoter (cPR) in the (−)repRNA or part of a dsRNA structure, could be unwound by Ded1p around the 3′ end, making the (−)repRNA “open”. The open ssRNA structure would facilitate (+)RNA synthesis by the tombusvirus p92 RdRp protein. GAPDH could facilitate (+)RNA synthesis via binding to a 3′ AU-rich sequence (indicated as a box) and also bringing in the p92 protein and positioning it over the cPR promoter as shown. Thus, Ded1p and GAPDH might play a complementary role, leading to synergistic effect on (+)RNA synthesis. In the absence of Ded1p, GAPDH could promote both 3′-terminal extension and (+)RNA synthesis by facilitating the positioning of p92 over the 3′ end.