Engineering Photosynthetic Pathways

Abstract
Improvements of metabolic reactions in photosynthetic pathways, and prospects for successfully altering photosynthetic carbon reduction (PCR) cycle in particular, have become possible through technologies developed during the last decade. This chapter outlines recent strategies and achievements in engineering enzymes of primary CO₂ fixations. We emphasize antisense approaches, attempts at engineering the chloroplast genome, and the transfer into C₃ species of reactions and enzymes typical for C₄ species or cyanobacteria. In addition, we point to the importance of studying the evolutionary diversity of enzymes in primary metabolism. The resulting transgenic lines then provide material suitable for precise flux control analysis. Discussed are enzymes of the photosynthetic reaction (PCR) cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), fructose 1,6-bisphosphatase (FBPase), sedoheptulose 1,7-bisphosphatase (SBPase), aldolase, and transketolase that exert control in a rate-limiting fashion. The PCR cycle, initiated by reactions that are catalyzed by RuBisCO, represents a major energy-consuming process in photosynthesis, justifying the large amount of research effort directed toward engineering this important enzyme. We also discuss progress in fine-tuning the two competing reactions catalyzed by RuBisCO, and in defining the roles and importance of PCR components, such as FBPase and SBPase. Lasting success is still elusive in improving crops by increasing primary productivity, but new tools have provided promising new avenues.

Key Words: RuBisCO, Photosynthetic carbon reduction cycle, Flux control analysis, Photorespiratory oxidation cycle, Relative specificity, RuBisCO-like protein, Enzyme engineering, Metabolic engineering, Chloroplast transformation, C₄-ization, Phosphoenolpyruvate carboxylase, Pyruvate Pi dikinase, NADP⁺ -malic enzyme.