Reverse self-splicing of the tetrahymena group I intron: implication for the directionality of splicing and for intron transposition.

Using short oligoribonucleotides as ligated exon substrates, we show that splicing of the Tetrahymena rRNA group I intron is fully reversible in vitro. Incubation of ligated exon RNA with linear intron produces a molecule in which the splice site sequences of the precursor are reformed. Reversal of self-splicing is favored by high RNA concentration, high magnesium and temperature, and the absence of guanosine. 5' exon sequences that can pair with the internal guide sequence of the intron are required, whereas 3' exon sequences are not essential. Integration of the intron into ligated exon substrates that have the ability to form stem-loop structures is reduced at least one order of magnitude over short, unstructured substrates. We propose that the formation of these structures helps drive splicing in the forward direction. We also show that the Tetrahymena intron can integrate into a beta-globin transcript. This has implications for transposition of group I introns.