The secondary structure of TERC, showing the mutations used in this study. The secondary structure domains are indicated. In the boxes the mutated bases are indicated by capital letters. The bases complementary to mutations C72G, GC107-8AG, and C408G are shown in italics. The secondary structure elements in the pseudoknot domain referred to in the text are indicated. In this in vivo system, the telomerase components must be translocated to the nucleolus and assembled into the functional complex for any activity to be detected.