Investigating the molecular basis for Rubisco acclimation to increasing temperature

HYPOTHESIS: Protein isoform composition of the multi-subunit Rubisco enzyme changes in response to environmental temperature. This opens the possibility that Rubisco engineering could be used to increase the geographical range of important crops and safeguard food security in the wake of global warming.

BACKGROUND: Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), the most abundant enzyme on earth, has a key function in photosynthetic conversion of inorganic CO2 into organic carbon compounds. This complex process is fundamental to life on earth and underpins all natural ecosystems and the human food chain. However, like all enzyme-driven processes, CO2 fixation is a temperature-sensitive process and an understanding of how enzymes in this pathway respond to rising global temperatures could have important implications in agriculture (and algal biotechnology). This project aims to establish the mechanism by which Rubisco acclimates to high temperatures through specific modifications to the enzyme complex.

Rubisco is a bifunctional enzyme with photosynthesis-driving carboxylase activity and the subversive oxygenase activity. The latter decreases photosynthetic efficiency in a temperature-dependent fashion, leading to reduced plant productivity. Plant Rubisco has eight large subunits encoded by the chloroplast gene rbcL and eight small subunits encoded by a family of nuclear rbcS genes. The chloroplast rbcL is a single-copy gene, which ensures production of identical large-subunit proteins. However, multiple copies of nuclear rbcS genes produce nonidentical small-subunit proteins. Expression of Arabidopsis rbcS genes changes under a range of temperatures. Additionally, different Mesembryanthemum crystallinum rbcS genes are disproportionately down-regulated during C3 to CAM switch. Furthermore, rubisco of Spinacia oleracea displays different kinetic profiles after acclimation at different temperatures. Different rbcS expression profiles and attendant Rubisco properties in plants under changing conditions led to the proposal that different rbcS gene-encoded small subunits have a role in fine-tuning Rubisco to acclimate to different conditions. This is reinforced by observations suggesting that plants exposed to significant changes in seasonal temperature may acclimate by shifting their photosynthetic optimum to match the external conditions. The study objective is a comparative study of the temperature sensitivity/resilience of photosynthetic systems, with particular focus on the Rubisco enzyme complex in a range of model organisms and crops.