Extreme variations in the ratios of non‐synonymous to synonymous nucleotide substitution rates in signal peptide evolution

Abstract

Nucleotide sequences encoding signal peptidcs from the precursors of α‐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 α‐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.