Crystal structure of the homo-tetrameric DNA binding domain of Escherichia coli single-stranded DNA-binding protein determined by multiwavelength x-ray diffraction on the selenomethionyl protein at 2.9-Å resolution

The crystal structure of the tetrameric DNA-binding domain of the single-stranded DNA binding protein from Escherichia coli was determined at a resolution of 2.9 Å using multiwavelength anomalous dispersion. Each monomer in the tetramer is topologically similar to an oligomer-binding fold. Two monomers each contribute three β-strands to a single six-stranded β-sheet to form a dimer. Two dimer–dimer interfaces are observed within the crystal. One of these stabilizes the tetramer in solution. The other interface promotes a superhelical structure within the crystal that may reflect tetramer–tetramer interactions involved in the positive cooperative binding of the single-stranded DNA-binding protein to single-stranded DNA.