The yeast recombination and repair proteins Rad1 and Rad 10 associate with a 1:1 stoichiometry to form a stable complex with a relative molecular mass of 190 kDa. This complex, which has previously been shown to degrade single-stranded DNA endonucleolytically, also cleaves supercoiled duplex DNA molecules. In this reaction, supercoiled (form I) molecules are rapidly converted to nicked, relaxed (form II) molecules, presumably as a result of nicking at transient single-stranded regions in the supercoiled DNA. At high enzyme concentrations, there is a slow conversion of the form II molecules to linear (form III) molecules. The Rad1/Radl0 endonuclease does not preferentially cleave UV-irradiated DNA and has no detectable exonuclease activity. The nuclease activity of the Rad1/Radl0 complex is consistent with the predicted roles of the RAD1 and RAD10 genes of Saccharomyces cerevisiae in both the incision events of nucleotide excision repair and the removal of nonhomologous 3′ single strands during intrachromosomal recombination between repeated sequences. In these pathways, the specificity and reactivity of the Rad1 / Radl0 endonuclease will probably be modulated by further protein-protein interactions.
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