Nucleic Acids Research, 1985, Vol. 13, No. 15 5471-5483
© 1985
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Mammalian DNA helicase
Department of Pharmacology and Biochemistry, University of Zürich-Irchel Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
*To Whom reprint requests should be sent
Received May 30, 1985. Accepted July 10, 1985.
A forked DNA was constructed to serve as a substrate for DNA helicases. It contains features closely resembling a natural replication fork. The DNA was prepared in large amounts and was used to assay displacement activity during isolation from calf thymus DNA polymerases a holoenzyme. One form of DNA polymerase
holoenzyme is possibly involved leading strand replication at the replication fork and possesses DNA dependent ATPase activity (Ottiger, H.-P. and Hüibscher, U. (1984) Proc. Natl. Acad. Sc1. USA 81, 3993-3997). The enzyme can be separated from DNA polymerase a by velocity sedimentation 1n conditions of very low 1onic strength and then be purified by chromatography on Sephacryl S-200 and ATP-agarose. At all stages of purification, DNA dependent ATPase and displacement activity profiles were virtually superimposable. The DNA dependent ATPase can displace a hybridized DNA fragment with a short single-stranded tail at its 3'hydroxyl end only 1n the presence of ATP, and this displacement relies on ATP hydrolysis. Furthermore, homogeneous single-stranded binding proteins from calf thymus as well as from other tissues cannot perform this displacement reaction. By all this token the DNA dependent ATPase appears to be a DNA helicase. It 1s suggested that this DNA helicase might act in concert with DNA polymerase
at the leading strand, possibly pushing the replication fork ahead of the polymerase.
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