Fig. 1. Schematic representation of the different prototypes of independently transcribed snoRNA-coding units. The boxes represent the snoRNA-coding regions, the stems and arrows indicate the Rnt1p substrates and cleavage sites, respectively, and the dots represent the cap structure. Poly- and monocistronic units are schematically represented in (A) and (B), respectively. In all cases studied so far, the 5′ cap is either present in the mature snoRNA (6) or it is removed by Rnt1p cleavage (1–5) (Chanfreau et al., 1998).