Family members of this domain are Type II site-specific deoxyribonuclease EC=3.1.21.4. The endonuclease SmaI Swiss:P14229, recognizes and cleaves the sequence CCCGGGG on DNA, yielding a blunt end scission [1]. It has been used for the diagnosis of neurogenic muscle weakness, ataxia and retinitis pigmentosa disease or Leigh's disease. Due to its specificity in recognizing the cleavage site, it is used in Leigh's disease to specifically eliminate the mutant mitochondrial DNA (mtDNA), which coexists with the wild-type mtDNA (heteroplasmy). Only the mutant mtDNA, but not the wild-type mtDNA, is selectively restricted by the enzyme. By delivering the SmaI gene fused to a mitochondrial targeting sequence, specific elimination of the mutant mtDNA was demonstrated, resulting in restoration of both the normal intracellular ATP level and normal mitochondrial membrane potential [2]. The same strategy has also been demonstrated retinitis pigmentosa (NARP), where a mutant mitochondrial DNA carrying a T8993G transversion has been targeted by using SmaI enzymes [3].

1: Sequence-specific DNA recognition by the SmaI endonuclease. Withers BE, Dunbar JC; J Biol Chem. 1995;270:6496-6504. PMID:7896784

2: Gene therapy for mitochondrial disease by delivering restriction endonuclease SmaI into mitochondria. Tanaka M, Borgeld HJ, Zhang J, Muramatsu S, Gong JS, Yoneda M, Maruyama W, Naoi M, Ibi T, Sahashi K, Shamoto M, Fuku N, Kurata M, Yamada Y, Nishizawa K, Akao Y, Ohishi N, Miyabayashi S, Umemoto H, Muramatsu T, Furukawa K, Kikuchi A, Nakano I, Ozawa K, Yagi K; J Biomed Sci. 2002;9:534-541. PMID:12372991

3: Selective elimination of mutant mitochondrial genomes as therapeutic strategy for the treatment of NARP and MILS syndromes. Alexeyev MF, Venediktova N, Pastukh V, Shokolenko I, Bonilla G, Wilson GL; Gene Ther. 2008;15:516-523. PMID:18256697