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 rR
NA can inhibit self-splicing of the Tetrahymena intron. We report that
splicing of Physarum intron 3 is also influenced by adjacent sequence
s. In this case, however, splicing is stimulated by the presence of a
second group I intron 24 nucleotides downstream. In vitro, intron 3 se
lf-splices 10-25-fold faster in transcripts containing both introns. I
n vivo, intron 3 is excised from pre-rRNA before intron 2 in the major
ity of transcripts, as judged by PCR amplification of processing inter
mediates. This is an unusual example in which self-splicing is enhance
d by the juxtaposition of two group I introns. This cooperative effect
may be mediated by a conformational change in the rRNA.