A second form of the single-strand specific endonuclease of Neurospora crassa which is associated with a double-strand exonuclease

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Abstract
A 2nd form of single-strand specific endonuclease, which is stable to heating up to 74.degree. C and does not bind strongly to phosphocellulose, was partially purified from extracts of mycelia of wild-type N. crassa. The endonuclease is associated with an equally heat-stable exonuclease which degrades linear but not circular double-stranded DNA and does not attack double-stranded RNA. The exonuclease probably also degrades single-stranded DNA. Both endonuclease and exonuclease activities are inhibited by 0.1-0.5 mM ATP. The exonuclease is preferentially inhibited by a variety of agents and preferentially inactivated by trypsin. A DNA-unwinding activity was also detected in the nuclease preparation. Protease(s) present in the nuclease preparation destroy the DNA-unwinding and exonuclease activities on incubation at 37.degree. C, but do not affect the endonuclease activity. However, the heat-stability and chromatographic properties of the endonuclease are affected by this treatment. The altered properties of the endonuclease are very similar to those of the single-strand specific endonuclease previously described. The combined nuclease activities of the unaltered preparation make up a putative recombination nuclease of N. crassa.

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