RNase P is a metalloenzyme which cleaves precursor tRNA. Two specific nucleotides, A90 and U93, are located in a strand of the P4 stem of the catalytic core of RNase P. This strand base-pairs with another RNA strand within the catalytic core, and the uracil and adenine nucleotides of interest are thought to 'stick out' to facilitate the binding of magnesium ions. We mutated these nucleotides to reveal their role in catalysis.