the self-splicing intron
/ self-SPLY-sing IN-tron /
We usually picture splicing as a big machine — the spliceosome — coming in to cut an intron out of an RNA. But some introns need no machine at all. Given the right conditions, they fold up and cut themselves out, then rejoin the surrounding exons, with no protein enzyme and no spliceosome involved. These are the self-splicing introns, and they were the surprise that revealed RNA can be a catalyst.
There are two main families. A group I intron folds into a precise shape and uses a free guanosine nucleotide as a chemical attacker to break the first splice site, then completes a second cut to release itself as a linear piece and join the exons. A group II intron does something strikingly familiar: it folds and uses an internal branch-point adenosine to attack the 5' site, forming a lariat — exactly like the spliceosome does. In both cases the RNA's own folded structure is the catalyst; the intron is acting as a ribozyme on itself.
This is more than a curiosity. The group II self-splicing introns look so much like spliceosomal introns, lariat and all, that they are widely seen as the ancestors of our spliceosome: an RNA that once spliced itself, over evolutionary time, recruited helper RNAs and proteins and let them take over the job, becoming the modern snRNP-based spliceosome. The self-splicing intron is thus a living fossil of the moment RNA did its own processing, and a key piece of evidence for the RNA world.
In the pond protozoan Tetrahymena, a group I intron in a ribosomal RNA precursor splices itself out in a test tube with only salts and a guanosine added — no protein present. This 1982 experiment by Tom Cech first proved RNA could catalyze its own splicing.
Some introns excise themselves with no protein enzyme — the first proof that RNA can catalyze.
Self-splicing is not the common case in your cells — the vast majority of human introns are removed by the spliceosome, and self-splicing introns are found mostly in organelles, bacteria, and some lower eukaryotes. Their importance is evolutionary and conceptual, not a description of everyday human splicing.