DNA polymerase III is the primary replicative polymerase of E. coli (prokaryotes), operating optimally at 37°C. It is not thermostable and is irreversibly denatured at PCR denaturation temperatures (~95°C). Taq polymerase (from Thermus aquaticus, optimum 72°C), Pfu polymerase (from Pyrococcus furiosus, optimum 75°C, has proofreading exonuclease activity), and Vent polymerase (from Thermococcus litoralis, also proofreading) are all thermostable and used in PCR.