A method has been developed to select for Escherichia coli mutants which lack certain nucleases. Using this method, two classes of mutants have been isolated: mutants deficient in endonuclease I, and mutants deficient in RNase I. Extracts from endonuclease I mutants had 0.1 to 0.3% of the RNA-sensitive wild-type endonuclease and, using single-stranded DNA, no RNA-resistant endonuclease. The location of the endonuclease I mutation on the bacterial genetic map was shown to be near st on the side opposite to mtl. Extracts from RNase I mutants had 0.1% of the magnesium-independent wild-type RNase or lower activity. Mutants deficient in endonuclease I or RNase I were essentially identical to wild-type with respect to their general biological properties
