Amino acid biases in the N- and C-termini of proteins are evolutionarily conserved and are conserved between functionally related proteins.

Analysis of the Arabidopsis thaliana, Saccharomyces cerevisiae, Mus musculus, Escherichia coli, Bacillus subtilis, Thermoplasma acidophilum, and Sulfolobus tokodaii genomes demonstrate that many amino acid biases occur at the N- and C-termini of proteins, a statistically significant number of these biases are evolutionarily conserved, and these biases occur in amino acids beyond the first and last five amino acids. Analyses designed to shed light on the mechanism causing amino acid biases suggest that in at least some cases the bias is caused by forces acting at the nucleic acid level. It is also demonstrated that in E. coli functionally related proteins show similar biases at the N- and C-termini suggesting that the mechanisms causing the biases are complex and in some cases are related to function.