On how hydrolysis at the 3' end is prevented in the splicing of a sequentially folded group I intron.

We propose a dynamic model for the competition between exon-exon ligation and 3'-end hydrolysis valid for sequentially folded pre-mRNA introns of group I. This model accounts for the delay in the formation of conserved helix P10 until the 5' exon has been cleaved, a requirement to prevent hydrolysis at the 3' end of the intron. The model is rooted on computer simulations whereby the pre-mRNA searches for its structure as it is being transcribed. Thus, a competing interaction, engaging the internal guiding sequence, occurs initially and prevents P10 from forming until the 3' end of the 5' exon is habilitated as a nucleophilic agent. It is further shown that a destabilization of the competing interaction invariably leads to 3' hydrolysis, crippling the splicing capability of the intron. The results may be probed by site-directed mutagenesis.