FIG. 6.

(A) Schematic diagram of a yeast three-hybrid system. The specific binding of MS2-RSV Mψ RNA to the Gag polyprotein would reconstitute the activity of transcriptional activator and lead to the expression of a reporter gene, lacZ. DB, DNA binding domain of LexA; AD, activation domain of Gal4; UAS, binding site for the transcriptional activator upstream of the reporter gene. (B) Diagram of the RNA hybrid expression vector. The RSV Mψ was introduced into the SmaI site downstream of two copies of the MS2 RNA coat protein binding sequence. MS2-Mψ hybrid RNAs were expressed from an RNase P promoter by using RNA polymerase III in S. cerevisiae. Thus, RNA hybrid contains both 5′ RNase P RNA leader and 3′ terminator sequences. Ura3 is a selectable gene. (C) Schematic structure of the hybrid RNA. It retains the 5′ MS2 stem-loop structure(s) and the 3′ end of RNase P RNA.