(A, top) Electrostatic surface model of Pur-alpha repeat I-II in complex with one ssDNA molecule (pink). Red and blue colorations of the surface indicate negative and positive electrostatic potentials, respectively. (B, top) Cartoon shows in addition the structure model of PUR repeat I (green) and II (blue). DNA interaction sites, seen in the crystal structure, are shown as red sticks and correspond to the residues highlighted in Figure 2A. (C, top) Model showing the most likely overall trajectory of dsDNA (pink) when bound to Pur-alpha repeat I-II. The double-strand is locally unwound and the two separated strands bind to the two opposing binding sites on the protein. (A, B, bottom) Representation as in (A, B, top), additionally showing the C-terminus connecting to PUR repeat III. PUR repeat III likely arranges at the opposing site of the nucleic-acid-binding region. (C, bottom) Schematic drawing of an intermolecular Pur-alpha dimer bound to dsDNA (pink). PUR repeat III (grey) mediates dimerization, potentially orienting both nucleic-acid-binding domains (repeat I, green and II, blue) to the dsDNA. There both PUR domains could unwind larger regions of the DNA.