Exploring purine N7 interactions via atomic mutagenesis: The group I ribozyme as a case study

A metal ion near the exogenous guanosine (G) in group I ribozymes. Residues are numbered according to the Tetrahymena group I intron. Distances are in Å. This metal ion in green is present in models derived from crystals of the Azoarcus and Twort group I ribozymes (Adams et al. 2004; Golden et al. 2005; Stahley and Strobel 2005, 2007) but not in those derived from crystal of the Tetrahymena group I ribozyme (Guo et al. 2005), probably because of the particular truncation of the crystallized construct. The contacts between this Mg²⁺ ion and the pro-R_P phosphoryl oxygens of residues 307 and 308 are supported by functional studies on the Tetrahymena group I ribozyme (Forconi et al. 2007).