FIG. 17.

Primary structure of the AddAB enzyme. The AddA protein (yellow) is structurally homologous to RecB; the N-terminal domain contains seven motifs (numbered) characteristic of SF1 helicases, and the C-terminal domain contains nuclease motifs. The position of a putative catalytic aspartate residue is marked “Nuc.” The AddB protein (magenta) shows weak homology to UvrD-like SF1 DNA helicases (which include RecB and AddA) and to the RecC protein in the regions indicated. A Walker A motif involved in the binding and hydrolysis of NTP, and equivalent to helicase motif 1, is found at the extreme N terminus. Nuclease motifs equivalent to those found in RecB and AddA are present toward the C terminus. The total number of amino acids (aa) in each polypeptide is indicated in parentheses.