An intron in the nuclear ribosomal DNA of Didymium iridis codes for a group I ribozyme and a novel ribozyme that cooperate in self-splicing.

We have discovered a unique group I intron-like insertion (DiSSU) in the nuclear small subunit ribosomal RNA gene of the myxomycete Didymium iridis. By sequence, DiSSU consists of a group I ribozyme at the 5' end, an open reading frame (ORF) in the middle, and a novel element at the 3' end. Intron RNA self-splices in vitro to yield ten major processed RNAs, including a full-length circle. The group I ribozyme can efficiently cleave at an internal processing site, which separates the group I ribozyme from the ORF. Surprisingly, deletion that remove the entire group I ribozyme do not impair cleavage at the 3' splice site, implying that the 3' element itself is a catalytic RNA. Deletions that remove portions of the 3' element prevent utilization of the 5' splice site, suggesting that this element cooperates with the upstream group I ribozyme in splicing. DiSSU appears to be the first example for the cooperative interaction of distinct ribozymes in RNA splicing.