DNA STRAND ANNEALING IS PROMOTED BY THE YEAST RAD52 PROTEIN

The Saccharomyces cerevisiae RAD52 gene plays a pivotal role in geneti c recombination, Here we demonstrate that yeast Rad52 is a DNA binding protein. To show that the interaction between Rad52 and DNA is direct and not mediated by other yeast proteins and to facilitate protein pu rification, a recombinant expression system was developed. The recombi nant protein can bind both single- and double-stranded DNA and the add ition of either Mg2+ or ATP does not enhance the binding of single-str anded DNA, Furthermore, a DNA binding domain was found in the evolutio nary conserved N terminus of the protein. More importantly, we show th at the protein stimulates DNA annealing even in the presence of a larg e excess of nonhomologous DNA. Rad52-promoted annealing follows second -order kinetics and the rate is 3500-fold faster than that of the spon taneous reaction. How this annealing activity relates to the genetic p henotype associated with rad52 mutant cells is discussed.