IN Saccharomyces cerevisiae, of the many genes required for excision repair of ultraviolet-damaged DNA, only RAD1 and RAD10 also function in genetic recombination1. Complex formation between the RAD1 and RAD10 gene products1 activates an endo-nucleolytic function that nicks single-stranded DNA2,3 and negatively supercoiled double-stranded DNA2. To characterize the recombination role of the proteins Radl and Radl0, we have investigated their interaction with the Holliday junction, a four-stranded structure that results from single-stranded crossover between two duplex DNA molecules and whose resolution is obligatory for the generation of mature recombinants. We show that Radl binds specifically to a Holliday junction and, in the presence of magnesium, catalyses the endonucleolytic cleavage of the junction. Junction cleavage by Radl proceeds sufficiently without Radl0, thus identifying Radl as the catalytic subunit of Radl/Radl0 endonuclease.
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