Abstract
A nuclease from N. crassa has been prepared to the hydroxylapatite stage of purification described by Rabin and Fraser (1). It degrades single stranded DNA in an essentially exonucleolytic process. It does not give any appreciable acid soluble material with double stranded DNA as substrate. This shows its high degree of specificity towards single stranded DNA. An extra activity which makes double strand breaks in the AT rich regions of DNA is described. A limited number of breaks are observed with calf or mouse DNAs, as judged by the decrease of sedimentation velocity in alkaline or neutral sucrose gradients. The number of breaks decreases with increasing the ionic strength. lambda DNA is broken in half at the AT rich middle region of the molecule. The circular replicative form of fd DNA is converted to linear pieces of a rather homogeneous length. It is concluded from these results that it is an endonuclease activity specific for d(A-T) rich regions in double stranded DNA.
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