Initiation of mating type interconversion. This protein is a site-specific endonuclease that cleaves a site in the mat locus on chromosome III. The double-strand break is followed by a unidirectional gene conversion event that replaces the information at the mat locus by information copied from either of the two homologous loci (HMR and HML) that reside at the extremity of the chromosome III. Endonuclease expression takes place in late G1 just before cells enter S phase. Rapidly degraded via the ubiquitin-26S proteasome system through two ubiquitin-conjugating enzymes UBC2/RAD6 and UBC3/CDC34. The metal-binding domain form zinc-fingers that are involved in binding of the DNA.