A metal cluster in the intron active site. Within the active site four-metal heteronuclear center (see Figure 3), three ions (M1-M2-K1) are reciprocally interconnected by single oxygen atoms. They form a bona fide cluster similar to organic phosphorus oxide clusters , with 13 vertices contributed by Mg2+, K+ and oxygen and phosphorus atoms. In the figure, magnesium (M1 and M2) is shown as yellow spheres, potassium (K1) as violet spheres, phosphorus (P) as white spheres with a purple border and oxygen (O) as red spheres. The subscript indexes are the number of the corresponding residue in the O.i. group II intron (the negative number is for the 5′-exon residue), while the superscript indexes represent the atom number. Continuous lines represent covalent bonds while dashed lines represent coordinative bonds. SP: scissile phosphate.