Cleavage experiments with deoxythymidine 3 ',5 '-bis-(p-nitrophenyl phosphate) suggest that the homing endonuclease I-PpoI follows the same mechanismof phosphodiester bond hydrolysis as the non-specific Serratia nuclease

We show here that two nucleases, Serratia nuclease and I-PpoI, with contras ting specificities, i.e. non-specific vs. highly sequence specific, share a structurally similar active site region with conservation of the catalytic ally relevant histidine and asparagine residues, On the basis of a comparis on of the available structures and biochemical data for wild type and mutan t variants of Serratia nuclease and I-PpoI we propose that both enzymes hav e a common catalytic mechanism, a proposition that is supported by our find ings that both enzymes accept deoxythymidine 3',5'-bis-(p-nitrophenyl phosp hate) as a substrate and cleave it in an identical manner. According to thi s mechanism a histidine residue functions as a general base and Mg2+ bound to an asparagine residue as a Lewis acid in phosphodiester bond cleavage, ( C) 1999 Federation of European Biochemical Societies.