Atomic-resolution Structure of Cyanobactrial Cytochrome C6 with an Unusual Sequence Insertion

During photosynthesis, electron transfer between two membrane-bound complexes, cytochrome b6f and photosystem I can be accomplished by the coppercontaining protein, plastocyanin, or the heme protein, cytochrome c6 [1]. Cytochromes c6 are water-soluble, low-spin heme-containing proteins involved in the highpotential (340-390 mV) electron transport chain. They are characterized by low molecular mass, 80-90 amino acid residues in the mature protein, and have a covalently bound heme group. Cytochrome c6 from the mesophilic cyanobacterium Synechococcus sp. PCC 7002 is unique among all known cytochromes c6 due to the presence of an unusual sevenresidue insertion, K44DGSKSL50. Furthermore, the present protein is unusual because of its very high content (36%) of the smallest residues (glycine and alanine) [2]. The crystallization experiments used the hangingdrop vapor-diffusion method. Red crystals (Fig. 1) suitable for X-ray analysis were obtained from 10 mM sodium Hepes, pH 6.2, and 2.2 M ammonium sulfate over a period of one week.