(color online) Molecular views of the active site with different (d)NTPs bound. Nucleic acids and amino acids are drawn with bonds and cartoon method, respectively. DNA and RNA are shown in red; protein and substrate NTP are shown in yellow. Magnesium ions are shown in red balls and the CA atom of Y639 in green ball. (a) The conformation in GTP-binding system after energy minimization. Representative conformations in systems bound with GTP, dGTP, ATP, CTP and UTP are shown in panels (b)–(f), respectively.