1. Introduction 68

2. Ferredoxin reduction by photosystem I 72

3. Ferredoxins 73

4. Ferredoxin[ratio ]thioredoxin reductase 73

4.1 Spectroscopic investigations of FTR 76

4.2 The three-dimensional structure of FTR from the cyanobacterium Synechocystis sp. PCC6803 77

4.2.1 The variable subunit 77

4.2.2 The catalytic subunit 81

4.2.3 The iron–sulfur center and active site disulfide bridge 82

4.2.4 The dimer 84

4.3 Thioredoxin f and m 85

4.4 Ferredoxin and thioredoxin interactions 86

4.5 Mechanism of action 88

4.6 Comparison with other chloroplast FTRs 92

5. Target enzymes 95

5.1 NADP-dependent malate dehydrogenase 95

5.1.1 Regulatory role of the N-terminal extension 97

5.1.2 Regulatory role of the C-terminal extension 99

5.1.3 Thioredoxin interactions 101

5.2 Fructose-1,6-bisphosphatase 101

5.3 Redox regulation of chloroplast target enzymes 103

6. Conclusion 103

7. Acknowledgements 104

8. References 104

A pre-requisite for life on earth is the conversion of solar energy into chemical energy by photosynthetic organisms. Plants and photosynthetic oxygenic microorganisms trap the energy from sunlight with their photosynthetic machinery and use it to produce reducing equivalents, NADPH, and ATP, both necessary for the reduction of carbon dioxide to carbohydrates, which are then further used in the cellular metabolism as building blocks and energy source. Thus, plants can satisfy their energy needs directly via the light reactions of photosynthesis during light periods. The situation is quite different in the dark, when these organisms must use normal catabolic processes like non-photosynthetic organisms to obtain the necessary energy by degrading carbohydrates, like starch, accumulated in the chloroplasts during daylight. The chloroplast stroma contains both assimilatory enzymes of the Calvin cycle and dissimilatory enzymes of the pentose phosphate cycle and glycolysis. This necessitates a strict, light-sensitive control that switches between assimilatory and dissimilatory pathways to avoid futile cycling (Buchanan, 1980, 1991; Buchanan et al. 1994; Jacquot et al. 1997; Schürmann & Buchanan, 2000).