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... 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 seven-residue 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. The structure of the reduced form of cytochrome c6 has been determined at 1.2 Å and refined to an R factor of 0.107. It reveals that the overall fold of the protein is similar to that of other class I c-type cytochromes despite the presence of the specific insertion. The insertion is located within the most variable region of cytochrome c6 sequence, i.e. between helic...