Neurospora endoexonuclease and its inactive (precursor?) form

Abstract

Two nuclease activities which were shown previously to copurify from extracts of log-phase Neurospora mycelia, a single-strand specific endonuclease activity (with DNA and RNA), and a strand nonspecific exonuclease activity (with DNA only) were found to be associated with a single polypeptide. The enzyme was therefore clasified as an endoexonuclease. In log-phase extracts, about 75% of this enzyme was found to exist in an inactive form which was activated in vitro either by endogenous phenylmethylsulfonyl fluoride sensitive proteinase(s) or by exogenous trypsin. The inactive form of endoexonuclease was purified 45-fold in 15% yield free of the active enzyme. On electrophoresis in 6M urea-polyacrylamide gels, it migrated at a much slower rate than the active enzyme, indicating that it is a less acidic and(or) larger protein than the active nuclease. The strong adsorption of this inactive enzyme on octyl-Sepharose suggests that the protein may have a relatively large hydrophobic domain. The protein may be a precursor of the active enzyme (a pronuclease) or a strong complex of enzyme with a proteinaceous inhibitor that is not dissociated in 6 M urea or during a variety of chromatographic procedures.