Bacteriophage T4 gene-59 helicase assembly protein is required for recombination-dependent DNA replication, which is the predominant mode of DNA replication in the late stage of T4 infection. T4 gene-59 helicase assembly protein accelerates the loading of the T4 gene-41 helicase during DNA synthesis by the T4 replication system in vitro. T4 gene-59 helicase assembly protein binds to both T4 gene-41 helicase and T4 gene-32 single-stranded DNA binding protein, and to single and double-stranded DNA. The structure of T4 gene-59 helicase assembly protein reveals a novel alpha-helical bundle fold with two domains of similar size, this being the N-terminal domain that consists of six alpha-helices linked by loop segments and short turns. The surface of the domain contains large regions of exposed hydrophobic residues and clusters of acidic and basic residues. This domain has structural similarity to members of the high-mobility-group (HMG) family of DNA minor groove binding proteins including rat HMG1A and lymphoid enhancer-binding factor, and is required for binding of the helicase to the DNA minor groove.