In the phosphoenolpyruvate carboxylases (PEPCs) of mesophiles, a glycine(Gly)-rich region is well conserved and involved in catalytic activity, whereas the sequence of the corresponding region is considerably divergent in PEPC of Thermus sp. In this study, the Gly-rich region of Thermus PEPC was converted to that of Escherichia coli PEPC. The resulting mutant enzyme was as heat stable as the wild-type. However, its optimum temperature was markedly decreased. Thus, the divergent Gly-rich region of Thermus PEPC contributes to its catalytic activity but not to stability at high temperature.