FIGURE 4.

Thermal inactivation of Rubisco purified from wild-type SS1 and small subunit mutants (14, 46). Rubisco (5 μg) in 0.5 ml of 50 mm Bicine (pH 8.0), 10 mm NaH¹⁴CO₃ (58 Ci/mol), 10 mm MgCl₂, and 1 mm dithiothreitol was incubated at each temperature for 10 min. The samples were then cooled on ice for 5 min, and carboxylase activity was initiated at 25 °C by adding 20 μl of 10 mm RuBP. The reactions were terminated after 1 min with 0.5 ml of 3 m formic acid in methanol. The activities were normalized to the specific activities measured after the 35 °C incubation (WT-SS1, 1.6 μmol/min/mg; S16A, 1.5 μmol/min/mg; P19A, 1.6 μmol/min/mg; Y32A, 0.3 μmol/min/mg; E43A, 0.2 μmol/min/mg; W73A, 0.4 μmol/min/mg; L78A, 0.1 μmol/min/mg; P79A, 0.6 μmol/min/mg; F81A, 0.6 μmol/min/mg; E92A, 0.4 μmol/min/mg).