Enhanced self-splicing of Physarum polycephalum intron 3 by a second group I intron

The third group I intron of Physarum polycephalum is found at the same site in the large subunit rRNA as the well-characterized intron from Tetrahymena thermophila. Formation of alternative structures in the rRNA can inhibit self-splicing of the Tetrahymena intron. We report that splicing of Physarum intron 3 is also influenced by adjacent sequences. In this case, however, splicing is stimulated by the presence of a second group I intron 24 nucleotides downstream. In vitro, intron 3 self-splices 10-25-fold faster in transcripts containing both introns. In vivo, intron 3 is excised from pre-rRNA before intron 2 in the majority of transcripts, as judged by PCR amplification of processing intermediates. This is an unusual example in which self-splicing is enhanced by the juxtaposition of two group I introns. This cooperative effect may be mediated by a conformational change in the rRNA.