Group I self-splicing intron in complex with its exons
Homo sapiens (Human)
A primary tRNA transcript often includes unnecessary nucleotides and some introns at the anticodon loops. Their removal via RNA splicing matures the tRNA. Most introns are removed in this process, while subsets of introns catalyze their own elimination from the primary tRNA. The group I self-splicing intron, which is a metalloenzyme, is the first discovered catalytic RNA intron. This discovery has revealed that not all enzymes are proteins.
Recently the crystal structure of the group I intron in the second step of splicing, the preceeding step of exon ligation, has been determined in a complex with both the 5'- and 3'-exons. The 5'-exon which has been cleaved from the intron in the first splicing step, connects to the internal guide sequence (IGS) of the intron through the formation of base-pairs. In addition, the 3'-OH of the 5'-exon is positioned for inline nucleophilic attack of the phosphate at the intron-3'-exon junction. The 3'-exon also forms base-pairs with the IGS. This structure includes two metal ions. Six phosphates converge to coordinate the two metal ions on opposite sides of the attacked phosphate. As a result of these interactions, the intron catalyzes splice site selection and exon alignment.
Protein Data Bank (PDB)
author: Yuko Tsuchiya