RNase T is a DEDDh-type DnaQ-like 3'-5' exoribonuclease E implicated in the 3' maturation of small stable RNAs and 23srRNA, and in the end turnover of tRNA. It contains 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. RNase T is related to the proofreading domain of DNA polymerase III. Despite its important role, RNase T is mainly found only in gammaproteobacteria. It is speculated that it might have originated from DNA polymerase III at the time the gamma division of proteobacteria diverged from other bacteria. RNase T is a homodimer with the catalytic residues of one monomer contacting a large basic patch on the other monomer to form a functional active site.