Catalytic component of endonuclease SceI (Endo.SceI), which cleaves specifically at multiple sites on mitochondrial DNA and produces double-stranded breaks. Endonuclease SceI (Endo.SceI) is a heterodimer of ENS2 and SSC1. The N-terminus is blocked. Belongs to the LAGLIDADG endonuclease family. S.cerevisiae displays strain polymorphism with regard to Endo.SceI endouclease activity. This is due to the mitochondrion-encoded, catalytic subunit ENS2, which exhibits strain polymorphism. It can be either present as continuous ORF in the mitochondrial genome (e.g. strain IAM 4274), present but disrupted by the insertion of GC clusters (e.g. strain D273-10B/A), or completely absent in the mitochondrial genome (e.g. strain S288c). In strain D273-10B/A, RF3 is interrupted by GC clusters, which introduce a shift of +1 base und thus break the frame, producing 4 overlapping ORFs. In strain D273-10B/A, RF3 is interrupted by GC clusters, which introduce a shift of +1 base und thus break the frame, producing 4 overlapping ORFs. In strain D273-10B/A, RF3 is interrupted by GC clusters, which introduce a shift of +1 base und thus break the frame, producing 4 overlapping ORFs. In strain D273-10B/A, RF3 is interrupted by GC clusters, which introduce a shift of +1 base und thus break the frame, producing 4 overlapping ORFs.