Cleavage of pre-tRNAs by the splicing endonuclease requires a composite active site

Splicing is required for the removal of introns from a subset of transfer RNAs in all eukaryotic organisms. The first step of splicing, intron recognition and cleavage, is performed by the tRNA-splicing endonuclease, a tetrameric enzyme composed of the protein subunits Sen54, Sen2, Sen34 and Sen15. It has previously been demonstrated that the active sites for cleavage at the 5' and 3' splice sites of precursor tRNA are contained within Sen2 and Sen34, respectively. A recent structure of an archaeal endonuclease complexed with a bulge-helix-bulge RNA has led to the unexpected hypothesis that catalysis requires a critical 'cation-pi sandwich' composed of two arginine residues that serve to position the RNA substrate within the active site. This motif is derived from a cross-subunit interaction between the two catalytic subunits. Here we test the role of this interaction within the eukaryotic endonuclease and show that catalysis at the 5' splice site requires the conserved cation-pi sandwich derived from the Sen34 subunit in addition to the catalytic triad of Sen2. The catalysis of pre-tRNA by the eukaryotic tRNA-splicing endonuclease therefore requires a previously unrecognized composite active site.