A novel c-type cytochrome transfers electrons between sulfite oxidase and cytochrome C(552) in the respiratory chain of Thermus thermophilus

We here describe a novel c-type cytochrome from the extreme thermophile Thermus thermophilus. N-terminal sequencing of the purified protein led to the identification of the corresponding gene TTHA1326. The 23 kDa cytochrome possesses two heme c binding sites and demonstrates a high sequence identity to cytochrome c552, the substrate of the ba3-type cytochrome c oxidase. Because of the low yield, we have succeeded in its recombinant production in E. coli with the simultaneous expression of the ccm genes involved in the maturation of cytochrome c in the same organism. We have generated milligram quantities of the holo-protein allowing the investigation of its properties and physiological function. There is no evidence that cytochrome c550 acts as an electron shuttle between the bc complex and Thermus cytochrome c oxidases. We have shown that, surprisingly, cytochrome c550 clearly mediates electrons to cytochrome c552. Further analysis of the putative operon encoding the protein led to the identification of a potential electron donor namely sulfite oxidase. In order to assess the subsequent electron transfer, sulfite oxidase (SO) TTHA1325 was produced recombinantly in E. coli and was shown to utilize the cytochrome c550 as the electron acceptor following oxidation of sulfite. To the best of our knowledge, this is the first characterization of the sulfite respiration system from a thermophilic bacterium.