During the 1970s, the pre-rRNA genes of the protozoan Tetrahymena thermophila were discovered to contain an intron. Careful searches failed to uncover even one pre-rRNA gene without the extra sequence, indicating that splicing is required to produce mature rRNAs in these organisms. In 1982, in vitro studies showing that the pre-rRNA is spliced at the correct sites in the absence of any protein provided the first indication that RNA can function as a catalyst, as enzymes do.
A whole raft of self-splicing sequences were subsequently found in pre-rRNAs from other single-celled organisms, in mitochondrial and chloroplast pre-rRNAs, in several pre mRNAs from certain E. coli bacteriophages, and in some bacterial tRNA primary transcripts. The self-splicing sequences in all these precursors, referred to as group I introns, use guanosine as a cofactor and can fold by internal base pairing to juxtapose the two exons that must be joined. As discussed earlier, certain mitochondrial and chloroplast pre-mRNAs and tRNAs contain a second type of self-splicing intron, designated group II introns.
The splicing mechanisms used by group I introns, group II introns, and spliceosomes are generally similar, involving two transesterification reactions, which require no input of energy (Figure 1). Structural studies of the group I intron from Tetrahymena pre-rRNA, combined with mutational and biochemical experiments, have revealed that the RNA folds into a precise three-dimensional structure that, like an enzyme, contains deep grooves for binding substrates and solvent-inaccessible regions that function in catalysis. The group I intron functions like a metalloenzyme to precisely place the atoms that participate in the two transesterification reactions adjacent to catalytic Mg2+ ions. Considerable evidence now indicates that splicing by group II introns and by snRNAs in the spliceosome also involves bound catalytic Mg2+ ions. In both group I and group II self-splicing introns, and probably in the spliceosome, RNA functions as a ribozyme, an RNA sequence with catalytic ability.
Fig1. Splicing mechanisms in group I and group II self splicing introns and in spliceosome-catalyzed splicing of pre-mRNA. The intron is shown in gray, the exons to be joined in red. In group I introns, a guanosine cofactor (G) that is not part of the RNA chain as sociates with the active site. The 3′-hydroxyl group of this guanosine participates in a transesterification reaction with the phosphate at the 5′ end of the intron; this reaction is analogous to that involving the 2′-hydroxyl groups of the branch-point As in group II introns and pre mRNA introns spliced in spliceosomes. The subsequent transesterification that links the 5′ and 3′ exons is similar in all three splicing mechanisms. Note that spliced-out group I introns are linear structures, unlike the branched intron products in the other two cases. See P. A. Sharp, 1987, Science 235:769.
