Description:
<jats:p>A single‐stranded DNA‐dependent ATP γ‐phosphohydrolase of <jats:italic>M</jats:italic><jats:sub>r</jats:sub> 56000 induced after infection of <jats:italic>Escherichia coli</jats:italic> cells with bacteriophage T4, probably the ATPase dependent on gene <jats:italic>dda</jats:italic> of the phage, was isolated. Studies on the enzyme show that in the presence of ATP and Mg<jats:sup>2+</jats:sup> ions it is capable of dissociating partially double‐stranded fd bacteriophage DNA into the single strands and that some 3000 enzyme copies are required to unwind the 6400‐nucleotides‐long DNA. Unwinding is inhibited by reducing the length of the single‐stranded portion of DNA to two nucleotides. In addition it can be inhibited by sulfhydryl reagents which block the ATPase or by trapping free enzyme molecules in the assay system. The results suggest that unwinding is initiated near the single‐stranded portion of the DNA and is driven by the ATPase. It further appears that the enzyme unwinds by adsorbing to the DNA. Affinity of the enzyme for double‐stranded DNA is not detectable by DNA binding assay.</jats:p>