Biochemical properties of thermophilic viral ATPases. The release of inorganic phosphate (Pi) during nucleotide hydrolysis by ATV p529 and p618 and STIV2 B204 was determined by the malachite green assay. The ATPase activity of B204 is reported as nmol × min−1 × mg−1, and that of p529 and p618 as nmol × min−1 × μg−1. (A) 0.015 mg/ml of p529 and (B) 0.01 mg/ml of p618 were incubated at 60°C with increasing amounts of ATP. Km values of 0.11 mM for p529, and 0.55 mM for p618 were determined. The maximum ATP hydrolysis velocity was 0.57 nmol × min−1 × μg−1 and 4.55 nmol × min−1 × μg−1 for p529 and p618, respectively. The inset shows a Hill plot, with a Hill coefficient of n = 1. The temperature dependence of the ATPase activity was analysed in the presence of 1 mM ATP for p529 (C) and p618 (D), and in the presence of 2 mM ATP at pH 4.5 for B204 (E). (F) The influence of pH on the activity of B204 ATP hydrolysis at 80°C. (G) Nucleotides hydrolysed by B204 at pH 4.5 80°C. Nucleotide hydrolysis depends on the presence of divalent cations. ATV p529 preferentially utilizes Mg2+(H), while p618 (I) and B204 (J) are less specific. A low background concentration of inorganic phosphate in panels H and I originates from a contamination of the protein preparation. The figure has been modified with permission from [7, 40, 41]. Copyright© American Society for Microbiology.