DEDDh 3'-5' exonuclease domain of RNA exonuclease 1, -3 and similar eukaryotic proteins
This subfamily is composed of RNA exonuclease 1 (REX1 or Rex1p), REX3 (or Rex3p), and similar eukaryotic proteins. In yeast, REX1 and REX3 are required for 5S rRNA and MRP (mitochondrial RNA processing) RNA maturation, respectively. They are DEDDh-type DnaQ-like 3'-5' exonucleases containing three conserved sequence motifs termed ExoI, ExoII and ExoIII, with a specific Hx(4)D conserved pattern at ExoIII. These motifs are clustered around the active site and contain four conserved acidic residues that serve as ligands for the two metal ions required for catalysis. REX1 is the major exonuclease responsible for pre-tRNA trail trimming and may also be involved in nuclear CCA turnover. REX proteins function in the processing and maturation of many RNA species, similar to the function of Escherichia coli RNase T.