Studies on sheep kidney nuclease. II. Limited digestion of phiX174 single-strand DNA.

Single strand DNA of bacteriophage phiX174 was digested by sheep kidney nuclease (nuclease SK) at 37 degrees C for 2-6 h and the digests were examined by 2.5% polyacrylamide gel electrophoresis. On this electrophoresis five bands were detected corresponding to the DNA fragments by scanning the gel at 260 nm. After these DNA fragments from the gel with the electrode buffer used for the electrophoresis, the molecular weights of fragments I, II, III, IV and V were determined by sucrose density gradient centrifugation. The molecular weights of these DNA fragments were 1.7 . 10(6), 1.2 . 10(6), 8.6 . 10(5), 4.5 . 10(5) and 2.8 . 10(5). From the melting curves and the base compositions of these fragments it was suggested that phiX174 single-strand DNA might have intramolecular double helical regions, which were hardly susceptible to attack by nuclease SK.