Purification and characterization of nuclease beta from Ustilago maydis

A nuclease highly active on single-stranded DNA has been purified 11,200-fold from Ustilago maydis. The enzyme has a sedimentation coefficient of 4.3 S and a Stokes radius of 36 A. The native form of the enzyme appears to be a single polypeptide chain of 68,000 daltons. The enzyme is fully active in the presence of chelating agents but is strongly inhibited by nucleotides. Maximal activity is seen at pH 6. The enzyme hydrolyzes linear DNA in an exonucleotytic fashion from the 5' end liberating 3'-mononucleotides and small oligonucleotides. Degradation of DNA takes place through a distributive mode rather than a processive mode. The enzyme is also active on both single-stranded and duplex circular DNA, dephosphorylates 5'-nucleotides, and hydrolyzes RNA.