Schematic of tRNA splicing pathways in different eukaryotes. (Top left and right) A typical unspliced pre-tRNA is shown in its accepted secondary structure, with the intron residues indicated by red circles except for the antepenultimate intron residue (dark red); residues N1–N37 of the 5′ exon indicated by light blue circles, except for N32 (white) and anticodon residues N34–N36, (dark blue); and residues N38–N73 indicated by purple residues. The antepenultimate intron residue pairs with N32 in the pre-tRNA. Arrows indicate sites of endonucleolytic cleavage of the pre-tRNA by the SEN/TSEN splicing complex. (Top left) Canonical pre-tRNA with a well-defined BHL motif. (Top right) One of several pre-tRNAs with a slightly different BHL motif. (Top center) A typical unspliced pre-tRNA is shown in linear form with the 5′ exon in blue, the intron in red, and the 3′ exon in purple. Endonucleolytic cleavage of the pre-tRNA results in formation of a 2′–3′-cyclic phosphate at the 3′ end of both the 5′ exon and and the intron, leaving a 5′-OH at the 5′ end of both the 3′ exon and the intron. (Left panel) In fungi, plants, and protozoa, the RNA 5′-kinase activity of the ligase Trl1 phosphorylates the 5′-OH end of the 3′-half-molecule using GTP, and the cyclic phosphodiesterase (CPDase) activity of Trl1 opens the 2′–3′ cyclic phosphate to a 2′-phosphate (green). Then the ligase activity of Trl1 joins the half-molecules by adenylylation of the 5′-phosphate of the 3′ exon and ligation to the 3′-OH of the 5′ exon, leaving a 2′ phosphate (green) at the splice junction. This 2′-phosphate is subsequently transferred to NAD by the 2′-phosphotransferase (Tpt1). (Right panel) In humans and metazoans, as well as in some archaea, the CPDase activity of the ligase RtcB opens the 2′–3′ cyclic phosphate of the 5′ exon to form a 3′-phosphate (green). Then, the ligase activity of RtcB joins the half-molecules by guanylylation of the 3′-phosphate of the 5′ exon and ligation to the 5′-OH of the 3′ exon, releasing GMP.
