An RNA-amino acid complex and the origin of the genetic code

The group I RNAs, of which the Tetrahymena ribosomal RNA intron is the most investigated example, catalyze their own splicing reactions. Splicing is initiated at a conserved site on the RNA that facilitates attack by exogenous guanosine (or its nucleotides) on the exon-intron junction. The guanosine site in the RNA's catalytic center also binds arginine, and is quite selective for the arginine side chain. This amino acid-RNA interaction is stereoselective, and L-arginine is preferred. Immediately at the site at which arginine binds there is one of only four RNA triplets in 92 group I RNA sequences: AGA/G and CGA/G. Thus the arginine contact site is within any of four different codons for arginine. Mutation of the conserved G in the middle of the triplet decreases affinity for the amino acid, showing that binding is sequence-specific. A pathway for the origin of the genetic code for arginine is suggested, based on the existence and properties of this sequence-specific, amino acid-specific RNA complex. The existence of a proto-ribosome related to the group I RNAs seems the most likely hypothesis. This notion is used to distinguish three periods in the development of the code. Restrained and exuberant hypotheses about the origin of the genetic code are distinguished, and some objections to these hypotheses are considered.