Forbidden penta‐peptides

There are 3,200,000 amino acid sequences of length 5 (penta‐peptides). Statistically, we expect to see a distribution of penta‐peptides that is determined by the frequency of the participating amino acids. We show, however, that not only are there thousands of such penta‐peptides that are absent from all known proteomes, but many of them are coded for multiple times in the non‐coding genomic regions. This suggests a strong selection process that prevents these peptides from being expressed. We also show that the characteristics of these forbidden penta‐peptides vary among different phylogenetic groups (e.g., eukaryotes, prokaryotes, and archaea). Our analysis provides the first steps toward understanding the “grammar” of the forbidden penta‐peptides.