Extracellular nucleases of pseudomonas BAL 31. I. Characterization of single strand-specific deoxyriboendonuclease and double-strand deoxyriboexonuclease activities

The culture medium of Pseudomonas BAL 31 contains endonuclease activities which are highly specific for single-stranded DNA and for the single-stranded or weakly hydrogen-bonded regions in supercoiled closed circular DNA. Exposure of nicked DNA to the culture medium results in cleavage of the strand opposite the sites of preexisting single-strand scissions. At least some of the linear duplex molecules derived by cleavage of supercoiled closed circular molecules contain short single-stranded ends. Single-strand scissions are not introduced into intact, linear duplex DNA or unsupercoiled covalently closed circular DNA. Under these same reaction conditions, φ{symbol}X174 phage DNA is extensively degraded and PM2 form I DNA is quantitatively converted to PM2 form III linear duplexes. Prolonged exposure of this linear duplex DNA to the concentrated culture medium reveals the presence of a double-strand exonuclease activity that progressively reduces the average length of the linear duplex. These nuclease activities persist at ionic strengths up to 4 M and are not eliminated in the presence of 5% sodium dodecyl sulfate. Calcium and magnesium ion are both required for optimal activity. Although the absence of magnesium ion reduces the activities, the absence of calcium ion irreversibly eliminates all the activities.