The trans-splicing group II intron c/d in the Oenothera mitochondrial nad1 gene is modified by RNA editing in domain 6. This C-to-U conversion generates the typical domain 6 structure, which prompted us to speculate that this RNA editing event might be essential for splicing. To test this hypothesis, we investigated the influence of unedited and edited sequences of the Oenothera intron on splicing in vitro. The stem of domain 6 of intron nad1-c/d was transplanted into the autocatalytic yeast intron aI5c, yielding chimeras with the genomic C and the edited U, respectively, 5' of the branchpoint A. When incubated under self-splicing conditions, only the edited chimera was released as a lariat, while the precursor with the genomically coded C remained inactive. Our results support the hypothesis that Oenothera group II intron nad1-c/d cannot be spliced from the primary transcript without previous editing in domain 6.