FIGURE 3.

Active xanthine transport activities of single Cys mutants. E. coli T184 harboring pT7–5/xanQ(C-less-BAD) with given mutations were grown aerobically at 37 °C in complete medium to mid-logarithmic phase, induced with IPTG (0.5 mm) for 2 h, normalized to 0.7 mg of protein per ml of cell suspension, and assayed for transport of [³H]xanthine (1 μm) at 25 °C. A, Initial rates of uptake measured at 5–20 s. Control values obtained from T184 harboring pT7–5 alone (0.018 nmol mg⁻¹ min⁻¹ on average) were subtracted from the sample measurements in all cases. Results are expressed as a percentage of the rate of Cys-less XanQ (1.9 nmol mg⁻¹ min⁻¹ on average) with standard deviations from three independent determinations. B, steady state levels of xanthine accumulation (reached at 1–10 min for most mutants). Control values obtained from T184 harboring pT7–5 alone (0.012 nmol mg⁻¹ on average) were subtracted from the sample measurements in all cases. Results are expressed as a percentage of the level of Cys-less XanQ (0.8 nmol mg⁻¹ on average) with standard deviations from three independent determinations.