Figure 3.

Number of catalytically competent dTTPs bound to 4A′ during steady-state dTTPase turnover. In a three-syringe experimental set-up (Materials and Methods), 4A′ (2 μM hexamer) was mixed with [α-³²P]dTTP (150 μM) for 60 s. Nonradiolabeled dTTP (7 mM) was added at 60 s, and the reactions were chased for 10–60 s prior to acid-quenching. As shown (▴), only one additional dTTP per hexamer was hydrolyzed to dTDP during the chase period. When reactions were not chased, the steady state hydrolysis of [α-³²P]dTTP (150 μM) continues at 0.14 ± 0.006 s⁻¹ (•).