The activity of the precursor of mature RNA whereby it catalyses its own (cis>) splicing. Group I introns (examples are Tetrahymena rRNA and hammerhead ribozymes) are self-spliced with the assistance of Mg²⁺ ions, an internal guide base sequence near the 5′’-end and other sequences that form internal base pairing to bring the structure into a reactive supersecondary structure; reaction proceeds by attack of the 3’-hydroxyl group of a guanosine (or guanine nucleotide) on the upstream exon-intron junction, which displaces the 3′’-end of the upstream exon; its newly generated 3’′-end then attacks the junction between the intron and the downstream exon to generate the spliced exons and the free intron. Group II intron self-splicing differs in that the attacking agent is the 2’-hydroxy group of an adenylate residue of the intron located near its junction with the downstream exon, and intermediates are the upstream exon and the downstream exon still ligated to the intron, whose 5’-guanosine hydroxy group is looped back to the internal adenylate residue and is held in a 2’,5’ phosphodiester bond. The looped intron (a lariat) is finally displaced by attack of the 3’-hydroxy group of the upstream exon on the intron-(downstream) exon junction. Spliceosomes use the group II mechanism, but as they include ancillary RNA and protein molecules, they are not in a strict sense, self-splicing. The phenomenon of trans-splicing refers to ligation of the upstream site to a downstream site which is not the closest in the linear sequence.