<p>This entry represents a 2-layer alpha/beta domain found at the N-terminal in homotetrameric tRNA-intron endonucleases [<cite idref="PUB00020493"/>], and as domains 1 (N-terminal) and 3 in the homodimeric enzymes [<cite idref="PUB00041082"/>]. tRNA-intron endonucleases (<db_xref db="EC" dbkey="3.1.27.9"/>) remove tRNA introns by cleaving pre-tRNA at the 5'- and 3'-splice sites to release the intron. The products are an intron and two tRNA half-molecules bearing 2',3' cyclic phosphate and 5'-hydroxyl termini [<cite idref="PUB00005788"/>]. These enzymes recognise a pseudosymmetric substrate in which 2 bulged loops of 3 bases are separated by a stem of 4 bp [<cite idref="PUB00030470"/>]. Although homotetrameric enzymes contain four active sites, only two participate in the cleavage, and should therefore, be considered as a dimer of dimers.</p>