snR81 box H/ACA RNP-catalyzed pseudouridylation. The secondary structure (5′ hairpin-H box Hinge-3′-hairpin-ACA tail) and the sequences of two internal loops (pseudouridylation pockets) of snR81 box H/ACA RNA, along with their substrate sequences (paired with the internal loop sequences) are shown. The two sets of four core proteins (Cbf5, Nhp2, Nop10, and Gar1) that bind to box H/ACA RNA are also depicted. Under normal conditions, the 5′ pseudouridylation pocket within the 5′ hairpin of snR81 RNA guides (via base-pairing) the modification of U2 snRNA at position 42 (indicated), and the 3′ pseudouridylation pocket within the 3′ hairpin guides the modification of 25S rRNA at position 1051 (indicated). Under stress (nutrient-deprivation), the 3′ pseudouridylation pocket not only guides the modification of 25S rRNA at position 1051, but also directs the modification of U2 snRNA at position 93 (indicated at far right). Dash (-), base-pairing; cross (×), mismatch.
