Structure of a thermophilic cyanobacterial b6f-type Rieske protein

The `Rieske protein' PetC is one of the key subunits of the cytochrome b6f complex. Its Rieske‐type [2Fe–2S] cluster participates in the photosynthetic electron‐transport chain. Overexpression and careful structure analysis at 2.0 Å resolution of the extrinsic soluble domain of PetC from the thermophilic cyanobacterium Thermosynechococcus elongatus BP‐1 enabled in‐depth spectroscopic and structural characterization and suggested novel structural features. In particular, both the protein structure and the positions of the internal water molecules unexpectedly showed a higher similarity to eukaryotic PetCs than to other prokaryotic PetCs. The structure also revealed a deep pocket on the PetC surface which is oriented towards the membrane surface in the whole complex. Its surface properties suggest a binding site for a hydrophobic compound and the complete conservation of the pocket‐forming residues in all known PetC sequences indicates the functional importance of this pocket in the cytochrome b6f complex.