Sequence requirements for branch formation in a group ll self-splicing lntron

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Abstract
Evidence is presented for the existence of a specific intronintron interaction, necessary for the formation of the branched product in the self-splicing reaction of a group II yeast mitochondrial intron. Trans-splicing reactions involving two RNA molecules (5'exon with covalently linked regions of intron and intron with covalently linked 3'exon) show that the presence of portions of intron domain I on the 51 molecule is necessary for the formation of branched products which are not seen with shorter 51 molecules. Modification/interference reactions show regions necessary for branch-formation and support a major role for specific regions of intron domain I. Further experiments, utilizing a truncated 31 molecule that is missing the conserved branchpoint nucleotide, indicate that domain VI may be required for a successful domain I interaction. A model for the formation of a proper branched structure includes implications for both cis and trans configurations.