Rubisco in planta kcat is regulated in balance with photosynthetic electron transport

Abstract

Site turnover rate (kcat) 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 b6f 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 b6f 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 CO2 response of Rubisco was measured in planta with a fast-response gas exchange system at maximum ribulose 1,5-bisphosphate concentration. Reaction site kcat was calculated from Vm and Rubisco content. Biological variation of kcat 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 kcat of 0.5 s−1 was measured in –FNR and –CBF plants containing sufficient Rubisco but having slow electron transport rates. Plotting kcat 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.