Mutational analysis indicate that mutants with junctionflanking C-GÁU or C-GÁC triple do not have higher À1 PRF activity than those of DU177 with U-AÁU triple occupying the same position. This is intriguing as one extra hydrogen bond is contributed by the stem C-G base pairing in the process of the U-AÁU to C-GÁU conversion. However, one stem-loop interbase hydrogen bond is also lost at the same time. In contrast to the C-GÁU triple, the C-GÁC triple can possess more than one Hoogsteen interaction depending on the pH condition (39) , and the À1 PRF efficiency may thus be restored to the level of U-AÁU triple. Therefore, it suggests the important role of tertiary stem-loop interactions in modulating the À1 PRF activity of DU177.