The Synechocystis ORF slr0201 product is involved in succinate dehydrogenase-mediated cyclic electron transfer around PSI

The Synechocystis sp. PCC 6803 open reading frame (ORF) slr0201 was originally annotated as heterodisulfide reductase B subunit (HdrB). The slr0201 encodes a 301-amino acid hypothetical protein with the predicted amino acid sequence significantly homologous to not only the HdrB from methanogenic bacteria, but also some novel succinate dehydrogenase C subunit (SdhC) found in Archaea and Campylobacter. Genetic manipulation via knocking-out approach created a{Delta} slr0201 mutant showing a{Delta} sdhB-like phenotype that was characterized by impaired succinate-dependent DCPIP reduction activities, reduced SDH-mediated respiratory electron transports, lower cellular contents of succinate and fumarate, slower KCN-induced increases in Chl fluorescence yield in the dark, and weak state 2/strong state 1 transitions, being indicative of a more oxidized PQ pool. In addition, slower re-reductions of the photosystem (PS) I reaction center P700 upon light-off were also monitored in the{Delta} slr0201, indicating functional involvements of Slr0201 in cyclic electron transfer around PSI. Both photoautptrophical and photomixotrophical growth rates of the{Delta} slr0201 strain resembled to those of the wild type, but substantial growth deteriorations occurred when arginine ([~]25 mM) or other two urea-cycle relevant amino acids (citrulline and ornithine) were added, which were attributed to generations and accumulations of certain hazardous metabolites. Based on the{Delta} sdhB-resembling phenotype, in conjunction with its high sequence similarities to some archaeal SdhC, we proposed that the slr0201 encodes a SDH function-relevant protein and is most likely the SdhC, a membrane anchoring subunit, which, while being genetically distinct from those in traditional bacterial SDH, belongs to the C subunit of novel archaeal SDH.