Formation of lariats and circles in self-splicing of the precursor to the large ribosomal RNA of yeast mitochondria.

Self-splicing of the precursor to large ribosomal RNA of yeast mitochondria leads not only to circles but also to lariats, structures that have not been observed before as products of self-splicing. Lariats were studied by electron microscopy after hybridization with an RNA complementary to the 3' half of the precursor. This leads to differentiation in at least two classes of lariats that vary in the position of the branch point. In all lariats the tail carries the 3' end, which suggests that a 5' end is used for branch formation with an internal nucleotide. The circles are formed from excised introns. They lack only three nucleotides encoded by mitochondrial DNA along with the 5'-terminal G added in the course of self-splicing. The diverse number of self-splicing products arising in vitro testifies to the considerable reactivity of this intron. The formation of lariats in an RNA catalyzed reaction may have implications for views on the mechanism of splicing of nuclear pre-mRNAs.