Thermostable nuclease is known to be an important pathogenic factor unique to Staphylococcus aureus and is commonly presumed to be one compound which has the same genetic origin.However,two ORFs in S.aureus genomes were predicted to encode nucleases.One is encoded the nuclease A (SNase) without a gene name (termed nucl); the other is encoded a thermonuclease (TNase) with a name of nuc (termed nuc2).In order to verify whether both two kinds of thermostable nuclease proteins coexpress really in S.aureus,the nucl and nuc2 genes were cloned and expressed in E.coli respectively,and both of the recombinant proteins showed thermostable nuclease activity in a toluidine blue-DNA assay.Further,a nucl-deleted mutant of S.