Schematic representation of the secondary structure of S. cerevisiae ITS1. Model according to Yeh et al. (1990) and Allmang et al. (1996). Cleavage sites A2, A4, A3, B1L, and B1S are indicated, including the conserved sequences known to be important for these cleavages. U residues in the remaining portion of the spacer downstream from A2 are shown as black circles; other residues, as open circles. The gray circles represent the nucleotides of the mature 5.8S rRNA sequence paired to ITS1. Potential, U-rich binding sites for Rrp5p, indicated as I–III, are over- or underlined. Shading indicates the extent of the previously tested deletion of site III (Van Nues et al. 1995), while the 5′ portion of the spacer known to be dispensable for processing (Henry et al. 1994; Van Nues et al. 1994) is represented by a gray line.
