Nucleotide composition of genes and hydrophobicity of the encoded proteins.

We find that true proteins are generally more hydrophobic than the corresponding hypothetical proteins encoded by the randomized gene nucleotide sequences. Furthermore, the protein hydrophobicity but not its gene nucleotide composition is conserved within evolutionary families of functionally related proteins. These two findings indicate that there is a general drift to modify gene nucleotide composition in the course of evolution. An inspection of codon usage in genes shows that the drift mainly increases the content of adenine at the expense of thymine.