Sequence analysis can be used to elucidate structural features responsible for enhanced thermal stability of proteins.To date,this simple approach has provided a lot of valuable knowledge for understanding the thermoadaptations that microorganisms have accumulated in their evolutionary history.Previous studies focused on psychrophilic adaptation generally have demonstrated that multiple mechanisms work together to increase protein flexibility and activity,as well as to decrease the thermostability of proteins.However,the relationship between high and low temperature adaptations remains unclear.To investigate this issue,we collected the available predicted whole proteome sequences of species with different optimal growth temperatures,and analyzed amino acid variations and substitutional asymmetry in pairs of homologous proteins from related species.We found that changes in amino acid composition associated with low temperature adaptation did not exhibit a coherent opposite trend when compared with changes in amino acid composition associated with high temperature adaptation.Although it is widely believed that the ancestors of bacteria and archaea were thermophilic,the evolutionary patterns of prokaryotes after their high temperature origins are still unclear.Our findings indicate that during their evolutionary histories the proteome-scale evolutionary patterns associated with prokaryotes exposed to low temperature environments were distinct from the proteome-scale evolutionary patterns associated with prokaryotes exposed to high temperature environments in terms of changes in amino acid composition of the proteins.