Purification of small polydisperse circular DNA of eukaryotic cells by use of ATP-dependent deoxyribonuclease

Small polydisperse circular (spc) DNAs of mouse thymocytes were purified by a procedure involving nitrocellulose column chromatography and the treatment of ATP-dependent DNase, which acts only upon linear DNA molecules. Nitrocellulose column chromatography prior to the enzyme treatment was essential because digestion of linear DNA duplexes by the enzyme was inhibited by the presence of concomitant single-stranded DNAs. Mitochondrial DNAs were eliminated by linearization with XhoI and digestion with ATP-dependent DNase. The size distribution of the purified spc DNA molecules ranged from 0.2 micron to more than 28 micron, with a mean length of 5.4 micron. Circular molecules of more than 0.4 micron long (or 1.2 kb) were free from the contamination of linear DNA fragments and pure enough to be cloned into plasmids.

Properties of the recB and recC gene products of Escherichia coli

The properties of the recB and recC gene products of Escherichia coli were studied using recB and recC gene-inserted plasmids. recB mutants and recC mutants lacked ATP-dependent DNase (recBC enzyme) but showed apparent recovery of enzyme activity on introduction of plasmids carrying the recB and recC gene, respectively. The ATP-dependent DNase was also constructed in vitro by mixing the recB and recC gene products encoded by the plasmids with the corresponding gene. Specific labeling of plasmid-encoded proteins by the maxicell method showed that the recB and recC gene products were 135,000 and 125,000 dalton proteins, respectively. These results suggest that the recB and recC genes are the structural genes of the beta and alpha subunits, respectively, of the recBC enzyme. A gene that encodes a protein of about 100,000 daltons was found to be located between the recB and recC genes. But the product of this gene was shown not to be included in the recBC enzyme.

Purification and properties of an acid deoxyribonuclease from rat small intestinal mucosa

An acid deoxyribonuclease has been purified from rat small intestinal mucosa by a procedure including ammonium sulfate fractionation, chromatographies on DEAE-cellulose, CM-cellulose and SE-Sephadex and finally isoelectric focusing. Polyacrylamide gel electrophoresis of the purified enzyme preparation showed one major and two minor bands, and the enzyme activity corresponded to one of the minor bands. The enzyme preparation was free of contaminating DNase I, DNase III, alkaline RNase, acid and alkaline phosphatases and nonspecific phosphodiesterase, but slight activities of DNase IV and acid RNase were detected. The enzyme did not require divalent cations for activity, had a pH optimum of 4.5 in 0.33 M sodium acetate buffer, and had an optimum temperature of 50 to 60 degrees C when assayed for 30 min. The rate of hydrolysis of native DNA was about 2.5-fold faster than that observed with denatured DNA. Its molecular weight was found to be 9.0 +/- 0.1. The enzyme catalyzes the endonucleolytic cleavage of native and denatured DNA, yielding oligonucleotides which have an average chain length of about 7, and which contain 3'-phosphoryl termini. The mode of action of the enzyme is double-strand scission.

Purification and properties of adenosine 5'-triphosphate-dependent deoxyribonuclease from Thermus thermophilus HB8

An ATP-dependent DNase has been purified from Thermus thermophilus HB8 by a procedure involving streptomycin precipitation, DEAE-cellulose chromatography, Sephadex G-200 gel filtration and heparin-agarose affinity chromatography. ATP-dependent DNase activity was separated into two distinct peaks, Peak A and Peak B, by heparin-agarose affinity chromatography. Each peak fraction was further purified by ATP-agarose affinity chromatography. Peak A and Peak B were eluted from an ATP-agarose column at 0.14 M and 0.28 M KCl, respectively, each as a single peak. Both enzyme activities require ATP and Mg2+ for the degradation of double- and single-stranded DNAs, and degrade denatured DNA about 1.5 times faster than native DNA. The two peaks are optimally active at 69 degrees C and have similar optimal pH ranges from 8.2 to 9.2. The two purified peaks were unstable on storage at -20 degrees C, but were remarkably stabilized by addition of 0.4 mg/ml bovine serum albumin. Ammonium sulfate strongly inhibits the activities of both peaks. The molecular weights of Peak A and Peak B are about 170,000 as estimated by glycerol gradient sedimentation. The average chain lengths of denatured DNA produced by Peak A and Peak B were 4.2 and 3.6, respectively, and the products were terminated by 5'-phosphoryl and 3'-hydroxyl groups. The limit-digested products of denatured DNA produced by Peak B consist of mono-, di-, tri-, tetra-, and pentanucleotides along with some larger fragments. The mode of action of both activities is processive and Peak A does not attack double-stranded circular DNA.