Crystallization and data collection Crystallization of purified andconcentrated Fdx4 (30 mg/mL) in 25 mM Tris/HCl + 150mM NaCl buffer at pH 8.5 wasperformed using the sitting drop vapor diffusion method by mixing 2 µL protein with 2 µL precipitantsolution and equilibrating these against a reservoir solution. After proteinand buffer administration, the plates were sealed and stored at roomtemperature.For the protein crystallization experimentsa home-made screening kit (Table A??) with 24 different buffer conditions wasused.For observing crystal formation, the plateswere monitored daily with a light microscope.(No further fine screening and adjustmentsaround/of the optimal crystallization conditions were done to optimize thecrystallization process for getting better crystals.) Diffraction data to ??? Å resolution for Fdx4were collected under cryogenic conditions at 100 K in a liquid nitrogen streamat a Rigaku Micromax 007HF, a microfocus rotating anode X-ray generatorproducing Cu-K? radiation (1.

5418 Å), on the imaging plate system mar345dtb.   Phase determination and structurerefinement The dataset was processed and scaled usingMOSFLM (indexed and integrated) and AIMLESS (scaled and merged).The Fdx4 crystal structure was solved bymolecular replacement with Phaser using another ferredoxin structure (PDB ID: 1M2A)determined at 1.5 Å resolution as the search model.Positional refinement of the model wascarried out using multiple rounds of refinement with REFMAC5 in the CCP4 softwaresuite followed by manual model rebuilding (improvement) using COOT.The quality of the final structure wasvalidated by SFCHECK. A summary of the data collection andrefinement statistics is listed in Table 1.All figures were drawn using PYMOL.(For calculation of a free R-factor, 7 % ofreflections from diffraction data sets were randomly assigned and excluded fromrefinement.)  

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