Thermus aquaticus is a thermophilic bacterium that grows between the temperatures of 50 and 80°C. Although the signal peptide binding groove of its Ffh M domain is lined with hydrophobic groups, only three of them are Met side chains. In contrast, the binding grooves of mesophilic organisms (those that live at normal temperatures) are lined with numerous Met side chains (11 in E. coli). In aIDition, the finger loop forming one wall of the binding groove is disordered in the X-ray structure of the E. coli M domain but ordered in that of T. aquaticus (Figure 12-48 in Voet; both proteins were crystallized at room temperature). Suggest a reason for these evolutionary adaptations in T. aquaticus
