An
endonuclease activity shown to be associated with Friend
leukemia virus has been characterized using double-stranded phi X174
DNA as substrate. In the presence of Mg2+, the
endonuclease activity was able to convert supercoiled
circular DNA duplexes to the relaxed form by introducing single-stranded nicks into the
DNA. Most of the nicked
DNA duplexes contained only one nick per strand, since unit length
DNA was the predominant species obtained when the nicked
DNA was analyzed by alkaline
sucrose gradient centrifugation. The regions into which the nick could be introduced were evenly distributed around the
circular DNA molecule. When Mn2+ was substituted for Mg2+ in the reaction mixture, the number of nicks introduced into
circular DNA duplexes by the virus associated
endonuclease was greatly increased. In contrast to circular duplexes, linear duplexes and
single-stranded DNA functioned poorly as substrates for the virus-associated
enzyme. The Friend
leukemia virus-associated
endonuclease activity is with respect to these characteristics very similar to the
endonuclease activity associated with the p32
protein of the avian myeloblastosis virus [1]. The molecular weight of the Friend
leukemia virus
endonuclease was estimated by gel filtration on a
Sephacryl S-200 column to be about 45 000.