Co-optimization of ribozyme substrate stacking and L-arginine binding.

A model of the Tetrahymena catalytic site predicts that nucleotide 262 (nt262) caps an RNA pocket in which nucleoside substrates and arginine-like competitive inhibitors reside. Here we show that substituted RNAs behave as if nt262 stacks on nucleoside substrates, supporting the model. The more frequent an nt262 is in natural sequences, the more reactive the corresponding Tetrahymena RNA is for both cognate and non-cognate nucleoside substrates. These more reactive RNAs with the majority nt262 also bind arginine more strongly, stereoselect more strongly in favor of L-arginine, and make a greater distinction between the somewhat similar side-chains of L-arginine and L-lysine. These parallel changes in interaction with nucleosides and arginine analogs seem best explained by stacking of the arginine's guanidino group under the nt262 base. One consequence is that selection for improved Tetrahymena catalysis with nucleosides should also yield an improved arginine site.