The DnaD is one of the primosomal proteins that are required for initiation and re-initiation of chromosornal DNA replication in Gram-positive bacteria. The DnaD protein is composed of two major structural domains: an N-terminal oligomerization domain and a C-terminal ssDNA binding domain. Here, we report the Crystal structure of the N-terminal domain (aa 1-128) of DnaD (DnaDn) of Geobacillus kaustophilus HTA426 at 2.3 angstrom resolution. The structure of DnaDn reveals an extended winged-helix fold, a typical double-stranded DNA binding motif as winged-helix proteins. DnaDn formed tetramers in the crystalline state, but the results of gel filtration chromatography further indicated that this domain of DnaD was a stable dimer in solution. The Structural analysis of DnaDn may suggest the binding sites for DNA and DnaB, and an assembly mechanism for Gram-positive bacterial DNA replication primosome. (c) 2008 Elsevier Inc. All rights reserved.