DEDDy 3'-5' exonuclease domain of family-A DNA polymerases, RNase D, WRN, and similar proteins
DEDDy exonucleases, part of the DnaQ-like (or DEDD) exonuclease superfamily, catalyze the excision of nucleoside monophosphates at the DNA or RNA termini in the 3'-5' direction. They contain four invariant acidic residues in three conserved sequence motifs termed ExoI, ExoII and ExoIII. DEDDy exonucleases are classified as such because of the presence of a specific YX(3)D pattern at ExoIII. The four conserved acidic residues serve as ligands for the two metal ions required for catalysis. This family of DEDDy exonucleases includes the proofreading domains of family A DNA polymerases, as well as RNases such as RNase D and yeast Rrp6p. The Egalitarian (Egl) and Bacillus-like DNA Polymerase I subfamilies do not possess a completely conserved YX(3)D pattern at the ExoIII motif. In addition, the Bacillus-like DNA polymerase I subfamily has inactive 3'-5' exonuclease domains which do not possess the metal-binding residues necessary for activity.