Isolation and characterisation of dhel II, a DNA helicase from Drosophila melanogaster embryos stimulated by Escherichia coli-type single-stranded-DNA-binding proteins.

We have purified a DNA helicase from Drosophila embryos by following unwinding activity during the purification of the cellular single-stranded DNA-binding protein dRP-A. This DNA helicase unwinds DNA 5' to 3', has a salt-tolerant activity, and has a preference for purine triphosphates as cofactors for the unwinding reaction. The purified enzyme consists of a single polypeptide of 120 kDa, which cosediments with the helicase activity. Sedimentation analysis suggests that this polypeptide exists as a monomer under high and low salt conditions. Dhel II is able to unwind long stretches of DNA, but with decreased efficiency. Addition of Escherichia coli-like single-stranded DNA-binding proteins stimulates the unwinding activity at least 10-fold on substrates greater than 200 nucleotides. In particular, the mitochondrial single-stranded DNA-binding protein isolated from Drosophila embryos is able to stimulate unwinding by dhel II. These properties show that the helicase described is different from another Drosophila helicase dhel I; it has thus has been classified as dhel II.