Nucleic Acids Research

Two new endonuclease activities, endonuclease B and endonuclease C, obtained from yeast nuclear preparations have been separated and partially characterized. Endonuclease B has a primary requirement for Mn which cannot be replaced by Mg or Ca , and makes single-strand scissions in 2+ 2+ double-stranded DNA. Endonuclease C is activated by either Mn or Mg , and makes single-strand scissions with Mg , while with Mn , scissions are made which result in double-strand breaks. Neither enzyme is active on denatured DNA, and both are inhibited by yeast RNA. Both enzymes exhibit pH optima at pH 5.0 and pH 7.2, and leave 5'-phosphoryl termini.