Abstract
Abstract Endonuclease I was purified from a lysate of Proteus mirabilis by dihydrostreptomycin sulfate precipitation, ammonium sulfate precipitation, ion exchange chromatography on diethylaminoethyl Sephadex, and isoelectric density gradient electrophoresis. Purified endonuclease I sediments as a 3 S molecule and degrades double stranded P. mirabilis chromosomal DNA to acid-soluble products. In the presence of Escherichia coli tRNA the enzyme sediments as 7.2 and 6 S molecular forms and degrades double stranded chromosomal DNA to fragments that sediment as 8 S DNA molecules. The 7.2 and 6 S forms of endonuclease I that can be generated by the addition of tRNA to the purified enzyme also are observed in the dihydrostreptomycin sulfate precipitate of the crude extract of the bacteria. The limited digestion of DNA in vitro to the 8 S fragments is not caused by insufficient enzyme or end product inhibition by the 8 S DNA products. The 8 S DNA fragments also can be extracted from intact P. mirabilis cells grown under certain conditions.
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