An intriguing aspect of our model is the possible regulation of the ''open'' and ''closed'' structure of the 39 UTR by eEF1Bc. Displacement of eEF1Bc bound to the 39-end by the viral replicase during (-)-strand synthesis could make the 39-terminus of the (+)strand RNA fold back into a 'closed' structure. This could prevent efficient re-utilization of the original (+)-strand template during TBSV replication, and the switch to efficient (+)-strand synthesis on the (-)RNA intermediate ( Figure 9B ). This model can also explain why the newly made (+)-strand RNA progeny will not enter the replication cycle in the absence of bound eEF1Bc within the originally-formed replicase complexes as observed previously in the CFE assay [20] . We propose that the new (+)RNA progeny need to leave the replicase complex, then bind to eEF1Bc in the cytosol and assemble new replicase complexes, followed by a new round of viral RNA replication. Thus, this model suggests that eEF1Bc plays a key role in regulation of the use of (+)-strand RNAs in TBSV replication ( Figure 9B ).