DNA nicking--closing activity from salmon testis.

Salmon testis is a good source of DNA nicking--closing (N--C) enzyme, as expected for rapidly proliferating cells. Partial purification was obtained but it resulted in an unstable form of N--C enzyme. Cruder fractions are useful for relaxing supercoiled DNA under conditions where other N--C enzymes are inactive. The enzyme is very tolerant of salt concentration, with activity still detectable at 0.6 M NaCl. It is also active at low temperatures with reasonable activity at 0 degrees C. However, at temperatures greater than 25 degrees C it is rapidly inactivated. Otherwise, its properties are similar to other eucaryotic N--C enzymes.