Self‐splicing of a group II intron in yeast mitochondria: dependence on 5′ exon sequences

It has been previously suggested that self‐splicing of group II introns starts with a nucleophilic attack of the 2′ OH group from the branchpoint adenosine on the 5′ splice junction. To investigate the sequences governing the specificity of this attack, a series of Bal31 nuclease deletion mutants was constructed in which progressively larger amounts of 5′ exon have been removed starting from its 5′ end. The ability of mutant RNAs to carry out self‐splicing in vitro was studied. Involvement of 5′ exon sequences in self‐splicing activity is indicated by the fact that a mutant in which as many as 18 nucleotides of 5′ exon remain is seriously disturbed in splicing, while larger deletions eliminate splicing entirely. Mutants containing a truncated 5′ exon form aberrant RNAs. One of these is a 425‐nucleotide RNA containing the 5′ exon as well as sequences of the 5′ part of the intron. Its 3′ end maps at position 374 of the 887‐nucleotide intron. The other is a less abundant lariat RNA probably originating from the remainder of the intron linked to the 3′ exon. We interpret this large dependence of reactivity of the intron on 5′ exon and adjoining intron sequences as evidence for base‐pairing interactions between the exon and parts of the intron, leading to an RNA folding necessary for splicing. Possible folding models are discussed.