Role of pigments and subunits in the cytochrome b6f complex of Synechocystis PCC6803

Abstract

From the cyanobacterium Synechocystis 6803 a highly purified monomeric cyt. b6f complex was shown to contain one chlorophyll a and the carotenoid echinenone, both bound exclusively by the cyt. b6 subunit. Reduction of cyt. b6 shifts, kinetically correlated, the Chl a spectrum. This is supported by the close proximity of Chl and heme bL in the modeled structure of the b6-subunit (in analogy to the bc1-complex). Also, the carotenoid spectrum is shifted, suggesting a close proximity to at least one of the two hemes. Accordingly, a light-triggered tuning of the Q-cycle seems possible. A directed mutant of Synechocystis impaired in echinenone synthesis incorporated another carotenoid, most likely ß-cryptoxanthin, instead. These results indicate an essential structural and functional role of the two pigments. Out of three different Rieske proteins encoded in the genome of Synechocystis, at least two have been identified in the isolated cyt. b6f complex of WT. All three Rieske-copies were characterized separately by double deletion mutants and by heterologous overexpression followed by reconstitution of the FeS-centers. The unusual characteristics of one Rieske protein suggest multiple cyt. b6f functions including various electron donors in Synechocystis. Characterization of a PetM-minus mutant using non-invasive fluorescence methods with whole cells revealed a non-essential function of this subunit. However, a regulating role for the stoichiometry and possibly assembly of different energy transducing proteins including PS1, phycobilisomes and a plastoquinol oxidase could be firmly established, suggesting a mediating role of the cyt. b6f complex for various electron transport routes.