A regulatory role of the PetM subunit in a cyanobacterial cytochrome b6f complex.

To investigate the function of the PetM subunit of the cytochrome b6f complex, the petM gene encoding this subunit was inactivated by insertional mutagenesis in the cyanobacterium Synechocystis PCC 6803. Complete segregation of the mutant reveals a nonessential function of PetM for the structure and function of the cytochrome b6f complex in this organism. Photosystem I, photosystem II, and the cytochrome b6f complex still function normally in the petM- mutant as judged by cytochrome f re-reduction and oxygen evolution rates. In contrast to the wild type, however, the content of phycobilisomes and photosystem I as determined from 77 K fluorescence spectra is reduced in the petM- strain. Furthermore, whereas under anaerobic conditions the kinetics of cytochrome f re-reduction are identical, under aerobic conditions these kinetics are slower in the petM- strain. Fluorescence induction measurements indicate that this is due to an increased plastoquinol oxidase activity in the mutant, causing the plastoquinone pool to be in a more oxidized state under aerobic dark conditions. The finding that the activity of the cytochrome b6f complex itself is unchanged, whereas the stoichiometry of other protein complexes has altered, suggests an involvement of the PetM subunit in regulatory processes mediated by the cytochrome b6f complex.