EXTREME VARIATIONS IN THE RATIOS OF NON-SYNONYMOUS TO SYNONYMOUS NUCLEOTIDE SUBSTITUTION RATES IN SIGNAL PEPTIDE EVOLUTION

Nucleotide sequences encoding signal peptides from the precursors of alpha-amylase/trypsin inhibitors from cereals are homologous to those corresponding to the precursors of thaumatin II and of plastocyanins. Non-synonymous (K(A)) and synonymous (K(S)) rates of nucleotide substitutions have been calculated for all possible binary combinations. Extreme variation in K(A)/K(S) ratios has been observed; from the 0.167 average found within the plastocyanin family to an average of 1.90 calculated for the inhibitors/thaumatin II transition. A similar calculation has been carried out for the signal peptide sequences of thionins, which are unrelated to those of the alpha-amylase/trypsin inhibitor family, and an average K(A)/K(S) of 0.12 has been obtained. This variation can be largely explained in terms of an empirical index of stability related to amino acid composition and seems to be independent of functional constraints.