Figure 3

Species distribution and characteristic amino-acid compositional signatures. Factorial plane representation and species colour coding are as in Figure 1. Amino acids (three letters) are discriminated according to statistically significant mean compositional differences (at the probability level of p < 0.001, see text and Additional file 3) between the three segregated groups (hyperthermophiles-thermophiles, HTH-TH; prokaryotic mesophiles-psychrophiles, PMES-PSYC; eukaryotes, EUK): amino acids in pale blue (Val, His, Ser) are characteristic for each of the three groups; amino acids in orange (Tyr, Glu, Asp, Thr, Gln) are characteristic for HTH-TH; amino acids in dark blue (Gly, Ile, Leu, Cys) are characteristic for EUK; amino acids in black (Lys, Asn, Phe, Met, Trp, Arg) show no significant differences between the three groups (Ala and Pro show only partial significant differences: between EUK and PMES-PSYC); for a given class, underlined amino acids correspond to high values steadily decreasing in the three groups downward (HTH-TH to PMES-PSYC and to EUK) whereas non-underlined amino acids correspond to low values steadily increasing in the same direction.