Catenanes of closed circular intracellular PM2 phage DNA

Type I DNA topoisomerases from mammalian cell nuclei interlock strains and promote renaturation of denatured closed circular PM2 DNA

Type I DNA topoisomerases from mouse ascites cell nuclei and from rat liver cell nuclei act on denatured viral closed circular PM2 DNA to produce molecules with a highly contracted structure as well as fully duplex non-supercoiled covalently closed circular molecules. Highly contracted DNA molecules contain a novel type of topological linkage in which a strand in one region of the double-stranded molecule passes between the strands in another region of the circular molecule one or more times. Since it is also found that the action of the topoisomerase promotes renaturation of complementary strands in denatured closed circular DNA, it is suggested that formation of contracted DNA structures proceeds through renatured, duplex intermediates with highly negative superhelix densities that contain small single-stranded regions.

Single-strand nicking of DNA in vitro by neocarzinostatin and its possible relationship to the mechanism of drug action

Neocarzinostatin, a protein antibiotic with anti-tumor activity was found to place single-strand scissions in DNA in an in vitro reaction. The drug's cutting activity was strongly dependent on the presence of 2-mercaptoethanol or dithiothreitol but some cutting did take place in the absence of reducing agent at very high drug levels and prolonged incubation. The requirement for reducing agents could not be replaced with NAD+, FAD, NADH or H2O2 and the strand-scission reaction was not affected by Mg2+, EDTA or intercalating agents. Similar profiles of heat-inactivation of neocarzinostatin were found whether activity was measured by the scission of DNA strand either in vitro or in HeLa cells treated with the drug. Furthermore, both of these parameters corresponded closely with the ability of the modified drug to inhibit DNA synthesis and growth of HeLa cells. By column isoelectric focusing it was shown that all four activities are associated with the same protein band (pH 3.28). From these data we conclude that the cytotoxic activity of neocarzinostatin and the nicking of DNA strands in vitro appear to reside in the same protein.

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-stranged 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 strang 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, 0X174 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.