Unveiling two distinct ribonuclease activities and a topoisomerase activity in a site-specific DNA recombinase.

The site-specific DNA recombinase Flp shows two types of RNA cleavage activities on hybrid DNA-RNA substrates. One targets the phosphodiester position involved in DNA recombination and follows a related mechanistic path. In this two-step reaction, first-strand scission is mediated by a nucleophilic attack of the scissile phosphodiester bond by the active site tyrosine of Flp. The resultant 3'-O-phosphoryl tyrosine bond is then attacked by the adjacent 2'-hydroxyl group. The second activity targets the immediately adjacent phosphodiester bond to the 3' side using a distinct mechanism. In this reaction, the vicinal 2'-hydroxyl directly attacks the phosphate group in a manner that is reminiscent of the pancreatic RNase mechanism. The Flp protein can also be shown to possess a topoisomerase-like activity.