Table 2.
Rubisco catalytic properties, leaf Rubisco content and photosynthetic performance
| Rubisco source | kcat (s−1) | KMRuBP (µM) | KCN2 (µM) | KC21%O2 (µM) | SC/O (M M−1) | Rubisco (µmol sites m−2) | Assimilation rate at 20 mbar CO2 (µmol m−2 s−1) | CO2 compensation point (Γ, µbar) |
|---|---|---|---|---|---|---|---|---|
| Tobacco | 3.1 ± 0.3 | 19 ± 3 | 9.7 ± 0.1 | 18.3 | 80 ± 2.6 | 21.9 ± 0.7 | 27.6 ± 0.5 | 55 ± 1 |
| [290 ± 3] | [548] | [2123 ± 69] | ||||||
| (n = 4) | (n = 3) | (n = 3) | (n = 3) | |||||
| CyLS plants | 9.8 ± 0.2 | 38 ± 1 | 158 ± 8 | 275 ± 8 | 55 ± 1 | 2.1 ± 0.3 | 24.7 ± 2.3 | 503 ± 69 |
| [4724 ± 249] | [8234 ± 240] | [1445 ± 19] | ||||||
| (n = 8) | (n = 9) | (n = 4) | (n = 3) | (n = 3) | (n = 3) | (n = 6) | (n = 3) | |
| CyLS-S1S2 plants | ||||||||
| Free Rubisco | 9.4 ± 0.4 | 36 ± 1 | 169 ± 14 | 285 ± 13 | n.d. | 1.5 ± 0.3 | 8.1 ± 0.4 | 553 ± 64 |
| [5063 ± 416] | [8533 ± 389] | |||||||
| (n = 7) | (n = 3) | (n = 4) | (n = 3) | (n = 3) | (n = 8) | (n = 3) | ||
| Isolated carboxysomes | 4.9 ± 0.3 | 59 ± 3 | n.d. | 248 ± 21 | n.d. | n.a. | n.a. | n.a. |
| [7430 ± 629] | ||||||||
| (n = 3) | (n = 6) | (n = 3) | ||||||
| Cyanobium cells | ||||||||
| Isolated carboxysomes | 9.5 ± 0.3 | 79 ± 2 | n.d. | 242 ± 21 | n.d. | n.a. | n.a. | n.a. |
| [7250 ± 629] | ||||||||
| (n = 3) | (n = 9) | (n = 3) | ||||||
| Broken carboxysomes | 10.4 ± 0.1 | 39 ± 2 | n.d. | n.d. | n.d. | n.a. | n.a. | n.a. |
| (n = 3) | (n = 6) | |||||||
Leaf samples of wild-type tobacco plants, CyLS and CyLS-S1S2 plants, grown at 2% CO2 (v/v), were extracted and prepared for Rubisco catalytic analysis as described in Methods. Tobacco kcat and KC values are from Sharwood et al.70 and KMRuBP is from Whitney et al.74. Free Rubisco from CyLS-S1S2 plants was obtained after high-speed centrifugation of crude leaf homogenates to pellet insoluble carboxysomes. Carboxysomes from both CyLS-S1S2 plants and Cyanobium cells were isolated as described in Methods and those of Cyanobium were broken to release free Rubisco by freeze–thaw treatment. Rubisco specificity values for CO2 (SC/O) were determined as described in Methods and are in vitro estimates from wild-type tobacco and CyLS plant Rubiscos. Data are means ± s.e.m. from (n) replicate measurements. Rubisco active site content of CyLS-S1S2 leaves was consistently lower than for CyLS leaves, but this was not statistically significant (P = 0.068; two-tailed, homoscedastic Student’s T test). Assimilation rates at 20 mbar CO2 were determined for independent leaf discs via membrane inlet mass spectrometry analysis. Values in brackets are in µbar, and bar bar−1 for SC/O
n.d. not determined, n.a. not applicable