The rate and specificity of a group I ribozyme are inversely affected by choice of monovalent salt.

The fifth intron of the COB gene of yeast mitochondria splices autocatalytically. The rate of splicing is increased by high concentrations of monovalent salts, but the choice of both cation and anion is significant: The smaller the cation in solution, the faster the reaction (the rate in K+ greater than NH4+ greater than Na+ greater than Li+). Chloride, bromide, iodide and acetate salts enhance autocatalytic processing, but sulfate salts do not and fluoride salts are inhibitory. The choice of monovalent salt affects the KM of the intron for guanosine nucleotide, implying an alteration in the affinity of the RNA for that substrate. Under optimal conditions (1M KCl, 50 mM MgCl2) the catalytic efficiency of this intron exceeds that reported for the ribosomal intron from Tetrahymena, but several side reactions occur, including guanosine-addition within the downstream exon. The site of addition resembles the 5' splice junction, but selection of this site does not involve the internal guide sequence of the intron.