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 C-terminal domain that consists of seven alpha-helices with short intervening loops and turns. The surface of the domain contains large regions of exposed hydrophobic residues and clusters of acidic and basic residues. The hydrophobic region on the 'bottom' surface of the domain near the C-terminal helix binds the leading strand DNA, whilst the hydrophobic region on the 'top' surface of the domain lies between the two arms of the fork DNA, allowing for T4 gene 41 helicase binding and assembly into a hexameric complex around the lagging strand.