The Synechocystis sp. PCC 6803 open reading frame slr0201 that is homologous to sdhC from Archaea codes for a protein

A hypothetical protein encoded by Synechocystis sp. PCC 6803 open reading frame slr0201shows high sequence similarity to the C subunit of a group of unusual succinate dehydrogenases found in some archaeal species. Slr0201 was originally annotated as HdrB, the B subunit of heterodisulfide reductase, but appears to be SdhC instead. This protein was overexpressed in E. coli by cloning the PCR-derived slr0201 open reading frame into a pET16b-based expression vector. The overproduced Slr0201 accumulated predominantly in inclusion bodies with an apparent molecular mass of 33 kDa. The protein contained at least one [2Fe-2S] cluster based on UV-visible absorbance and CD spectra and EPR spectroscopy, in conjunction with stoichiometric analysis of protein-bound iron and sulfur content. Redox titration showed a midpoint potential (Em) of + 17 mV at pH 7.0, which is consistent with Slr0201 serving a role in transferring electrons between succinate and plastoquinone. Slr0201 was also overproduced in Synechocystis sp. PCC 6803 by introducing an additional, His-tagged slr0201 into the Synechocystis genome replacing psbA3, creating the slr0201+-His overexpression strain. Immunoblot analysis shows that Slr0201 is membrane-associated in the wild type. However, in the Slr0201+-His strain, immunoreaction occurred in both the membrane and soluble fractions, possibly as a consequence of processing near the N-terminus. The results obtained with Slr0201 are discussed in the light of one of the cyanobacterial SdhB subunits, which shares redox commonalities with archaeal SdhB.