The thermostability of organisms and proteins is of considerable fundamental and biotechnological interest1-3. While most bacteria grow at temperatures ranging between 20 and 40°C, many archaea and a few bacteria require much higher temperatures to grow, such as Pyrococcus, capable of withstanding more than 100°C. It is tempting to expect that such different lifestyles should be imprinted in the genome and might already be detectable in the most basic feature of protein structure, i.e. the amino-acid composition. Here we explore this hypothesis using two approaches in parallel. First we compared the water accessible surfaces of 50 proteins from a mesophile bacterium, Escherichia coli, with those of 8 proteins from a hyperthermophile archaea, Pyrococcus. Second, we performed proteome composition comparisons using the 21 available genomes to date. The results point out a predominance of charged residues at the thermophilic protein surface, at the expense of polar (non charged) residues. At the proteome level, the main and only statistically significant effect is a large increase of glutamic amino-acids at the expense of glutamines. Overall, the amount of charged minus polar amino-acids in the proteome appears to be the best index of the thermophilic lifestyle.

[1] Goldman, A. How to make my blood boil. Structure 3,1277-1279 (1995).

[2] Vogt, G., Woell,S. and Argos, P. Protein thermal stability, hydrogen bonds and ion pairs. J.Mol.Biol. 269, 631-643 (1997).

[3] Richards,F.M., Protein stability: still an unsolved problem. Cell. Mol. Life Sci. 53, 790-802 (1997).