Evidence for remnants of an ordered codon sequence and a restricted codon composition in selected proteins

Abstract

For some 20 proteins the mRNA codon base sequence inferred from the amino acid sequence shows remnants of a regular pattern of the triplet mid-base. The repeat is a 4-triplet unit, but the identity of one specific position in the unit is normally varied. It is assumed the evolutionary forerunners of these proteins were repeat tetrapeptides that also showed this variability. Many of these proteins show a high incidence of guanine as triplet first base and are rich in aspartate and glutamate residues, and a low incidence of guanine as the mid-base. This is seen as resulting from an altered, more restricted, codon availability. It is postulated that for these proteins the triplet first base was once an obligatory guanine, and genetic information was restricted to the triplet mid-base. When a 3-base genetic code became effective intense mutational activity would introduce a second phase in the design of protein sequences.