Rubisco in planta kcatis regulated in balance with photosynthetic electron transport

Site turnover rate (k cat ) of Rubisco was measured in intact leaves of different plants. Potato (Solanum tuberosum L.) and birch (Betula pendula Roth.) leaves were taken from field-growing plants. Sunflower (Helianthus annuus L.), wild type (wt), Rubisco-deficient (–RBC), FNR-deficient (–FNR), and Cyt b 6 f deficient (–CBF) transgenic tobacco (Nicotiana tabacum L.) were grown in a growth chamber. Rubisco protein was measured with quantitative SDS-PAGE and FNR protein content with quantitative immunoblotting. The Cyt b 6 f level was measured in planta by maximum electron transport rate and the photosystem I (PSI) content was assessed by titration with far-red light. The CO 2 response of Rubisco was measured in planta with a fast-response gas exchange system at maximum ribulose 1,5-bisphosphate concentration. Reaction site k cat was calculated from V m and Rubisco content. Biological variation of k cat was significant, ranging from 1.5 to 4 s -1 in wt, but was >6 s -1 at 23 °C in –RBC leaves. The lowest k cat of 0.5 s -1 was measured in –FNR and –CBF plants containing sufficient Rubisco but having slow electron transport rates. Plotting k cat against PSI per Rubisco site resulted in a hyperbolic relationship where wt plants are on the initial slope. A model is suggested in which Rubisco Activase is converted into an active ATP-form on thylakoid membranes with the help of a factor related to electron transport. The activation of Rubisco is accompanied by the conversion of the ATP-form into an inactive ADP-form. The ATP and ADP forms of Activase shuttle between thylakoid membranes and stromally-located Rubisco. In normal wt plants the electron transport-related activation of Activase is rate-limiting, maintaining 50–70% Rubisco sites in the inactive state.