The present invention provides an endonuclease I or enzymatically active fragment thereof wherein said endonuclease I has the sequence of SEQ ID No. 4 or a sequence which is at least 70% identical thereto and wherein the amino acid residue which is immediately N-terminal of the FYCGC pentapeptide motif has been substituted with a residue which is negatively charged as well as nucleic acid molecules encoding these enzymes and methods of removing contaminating polynucleotides from a sample using these enzyme.
