It is intriguing that an RNA motif derived from the RNA component of telomerase can act as a À1 PRF stimulator. This could be explained by the existence of a common structural feature such as a specific RNA-protein contact, which is needed for a telomerase as well as a À1 PRF stimulator to function properly. Interestingly, telomerase RNA subunit has been identified in Marek's disease virus (41) . Perhaps, different viruses might pick up this motif during evolution and modified it to fit replication purpose, including the tuning of À1 PRF efficiency through mutation (30) . Nevertheless, it is informative to note that specific 2 0 OH groups protruding from the triple helix of DU177 have been shown to contact with the protein component of telomerase (42) . Therefore, the bimolecular pseudoknot provides a workable platform to explore the possibility of specific stimulator-ribosomal helicase interaction during À1 PRF as suggested previously (22) . In addition, it should be useful in probing the geometry of the mRNA entry channel and the helicase activity of eukaryotic ribosomes. Finally, the regulation of À1 PRF in-trans shown here raises the possibility that Figure 6 . An intermolecular C-GC major-groove base-triple interaction can replace an intermolecular U-AU triple. The C-GÁC majorgroove base-triple interaction reconstituted in the bimolecular pseudoknot construct is typed in gray, and the other designations are the same as those in Figure 5 .