 <unk> @-@ dependent kinases ( <unk> ) , which modify the activity of other proteins by adding phosphate groups to ( that is , <unk> ) them , are important regulators of homologous recombination in eukaryotes . When DNA replication begins in budding yeast , the cyclin @-@ dependent kinase <unk> begins homologous recombination by <unk> the Sae2 protein . After being so activated by the addition of a phosphate , Sae2 uses its endonuclease activity to make a clean cut near a double @-@ strand break in DNA . This allows a three @-@ part protein known as the MRX complex to bind to DNA , and begins a series of protein @-@ driven reactions that exchange material between two DNA molecules . 
