The Meaning of a Redundant Codon: There is Protein Folding Information in Nucleic Acids in Addition to the Genetic Code

Abstract

All the information necessary for protein folding is supposed to be present in the amino acid sequence. It is still not possible to provide specific ab initio structure predictions by bioinformatical methods. It is suspected that additional folding information is present in protein coding nucleic acid sequences, which is not represented by the known genetic code. Nucleic acid subsequences comprising the 1st and/or 3rd codon residues in mRNAs express significantly higher free folding energy (FFE) than the subsequence containing only the 2nd residues (p<0.0001, n=81). This periodic FFE difference is not present in introns and therefore it is a specific physico-chemical characteristic of coding sequences and it might contribute to unambiguous definition of codon boundaries during translation. The FFE in the 1st and 3rd residues is additive, which suggests that these residues contain a significant number of complementary bases and contribute to selection for local RNA secondary structures in coding regions. This periodic, codon-related structure-forming of mRNAs indicates a connection between the structure of exons and the corresponding (translated) proteins. The folding energy dot plots of RNAs and the residue contact maps of the coded proteins are indeed similar. Residue contact statistics using 81 different protein structures confirmed that amino acids that are coded by partially reverse and complementary codons (Watson-Crick (WC) base pairs at the 1st and 3rd codon positions and translated in reverse orientation) are preferentially co-located in protein structures. Exons are distinguished from introns and codon boundaries are physico-chemically defined by periodically distributed FFE differences between codon positions. There is a selection for local RNA secondary structures in coding regions and this nucleic acid structure resembles the folding profiles of the coded proteins. The preferentially (specifically) interacting amino acids are coded by partially complementary codons, which strongly supports the connection between mRNA and the corresponding protein structures and indicates that there is protein folding information in nucleic acids that is not present in the genetic code. This might give some additional explanation of codon redundancy.