Figure 9. Model on the roles of co-opted cellular helicases in asymmetrical replication of tombusviruses.

Cis-acting replication elements present at the 3′ and 5′ ends of the viral (−)RNA are recognized by cellular helicases to locally open the viral dsRNA replication form. Namely, the eIF4AIII-like helicases unwind the dsRNA structure within the RIII(−) REN, while the DDX3-like helicases open up the dsRNA structure within RI(−) (as shown schematically). Long-range RNA-RNA interaction between the “bridge” and the cPR region (promoter sequence at the 3′ end) in the (−)RNA is proposed to lead to circularization of the viral (−)RNA, which is still part of the dsRNA. This RNA structure is suggested to facilitate the transfer of the viral p92^pol from the 5′ end of the (−)RNA [after completion of the (+)RNA strand synthesis in the previous round] back to the 3′ cPR in the (−)RNA for a new round of (+)RNA strand synthesis. Another co-opted cellular protein, GAPDH (called Tdh2/3 in yeast) is likely involved in this process by facilitating the binding of p92^pol to the 3′end of the (−)RNA. Altogether, the re-use of the viral p92^pol multiple times on the same (−)RNA template could result in production of multiple (+)RNA progeny, resulting in the characteristic asymmetric viral RNA replication. Note that it is also possible that p92^pol is standing still, while the RNA template is moved during RNA synthesis within the membrane-bound VRCs.