Life on Earth almost exclusively depends on the reduction of inorganic atmospheric carbon dioxide (CO2) into organic molecules, via photosynthesis. Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) catalyzes the first irreversible enzymatic step of this process: the addition of CO2 to a 5-carbon molecule (ribulose-1, 5-bisphosphate, RuBP). Rubisco's abundance belies its kinetic shortcomings, which include a slow catalytic rate and a tendency to confuse its substrate, CO2, with O2. Biochemical and structural limitations constrain the evolution of the enzyme, but our understanding of the structure-function relationships of Rubisco is in its infancy. In land plants and green algae, Rubisco is a multimer of eight large and small polypeptide subunits (LSu and SSu, respectively). Because it houses the catalytic site, the LSu has been widely researched and characterized. Conversely, the role and origin of the SSu is unclear, although its structure and molecular biology are both well characterized. In this thesis I explore the impact of an altered SSu complement on Rubisco activity and photosynthetic performance. In Chapter 1, I show that temperature-induced changes in Rubisco performance can have a significant impact on photosynthetic carbon gain. These changes are associated with altered rbcS gene expression in some species, and in chapter 2 I demonstrate that the expression of two rbcS genes vary with growth temperature, but not CO2 in A. thaliana. In Chapter 3, I show that these changes in rbcS gene expression are associated with differences in SSu protein accumulation, and that changes in the SSu complement from warm-grown plants are associated with the production of a Rubisco that is more specific for CO2 at elevated growth temperatures. In Chapter 4, using whole plant gas-exchange, I show that SSu associated kinetic differences improve photosynthetic nitrogen use efficiency in the growth environment, likely by producing a more efficient enzyme. In total, this work characterizes the evolution of the genes, peptides, and function of the SSu of Rubisco, and will expand our understanding behind the evolution of the world’s most abundant enzyme.
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