Extreme variations in the ratios of non-synonymous to synonymous nucleotide substitution rates in signal peptide...

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 (KA) and synonymous (KS) rates of nucleotide substitutions have been calculated for all possible binary combinations. Extreme variation in KA/KS 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 KA/KS 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.