The DNA helicase activities of Rad3 protein of Saccharomyces cerevisiae and helicase II of Escherichia coli are differentially inhibited by covalent and noncovalent DNA modifications.

Rad3 protein of Saccharomyces cerevisiae is a DNA-dependent ATPase that acts as a DNA helicase on partially duplex substrates. Rad3 protein is required for damage-specific incision of DNA during the nucleotide excision repair (NER) pathway in yeast. Helicase II of Escherichia coli is also a DNA helicase, but it is involved in postincision events in NER. Previous investigations have demonstrated that the DNA helicase activities of Rad3 protein and helicase II are both inhibited by DNA damage. In the present study we have compared the response of yeast Rad3 protein and E. coli helicase II to a broad spectrum of DNA modifications. The Rad3 helicase activity is considerably more sensitive to ultraviolet radiation damage and cisplatin adducts in DNA than to drugs that interact noncovalently with duplex DNA. Conversely, E. coli helicase II is highly sensitive to noncovalent DNA modifications but less sensitive than Rad3 protein to ultraviolet radiation damage or cisplatin adducts. We also show that Rad3 protein and helicase II differ in their ability to form stable protein-DNA complexes at sites of DNA damage. Hence, DNA helicases that catalyze distinct steps in NER respond differently to chemical and conformational states of the DNA substrate. The observation that Rad3 protein is particularly sensitive to covalent but not noncovalent alterations in DNA structure is consistent with the hypothesis that this enzyme may have adopted a highly specialized role in damage-specific recognition during NER.