RNA affinity for molecular L-histidine; genetic code origins.

Selection for affinity for free histidine yields a single RNA aptamer, which was isolated 54 times independently. This RNA is highly specific for the side chain and binds protonated L-histidine with 10(2)-10(3)-fold stereoselectivity and a dissociation constant (K(D)) of 8-54 microM in different isolates. These histidine-binding RNAs have a common internal loop-hairpin loop structure, based on a conserved RAAGUGGGKKN(0-36) AUGUN(0-2)AGKAACAG sequence. Notably, the repetitively isolated sequence contains two histidine anticodons, both implicated by conservation and chemical data in amino acid affinity. This site is probably the simplest structure that can meet our histidine affinity selection, which strengthens experimental support for a "stereochemical" origin of the genetic code.