Endonuclease involved in group I intron homing. Recognizes and cleaves a 19-24 bp palindromic DNA site. Binds 3 Mg(2+) ions per homodimer. The enzyme can also utilize Mn(2+) or Co(2+), but has lower cleavage activity with Ni(2+) or Zn(2+). Ca(2+) Co(2+) give no enzyme activity. Homodimer. Belongs to the LAGLIDADG endonuclease family.