Endonuclease involved in intron homing. Recognizes a degenerate sequence of 17-19 bp to produce a staggered cut 5 bp downstream from the CeLSU.5 intron insertion site. Binds 2 magnesium ions per subunit. A third magnesium ion may be bound between subunits. Homodimer. Belongs to the LAGLIDADG endonuclease family.