Genetic and immunological analyses of the cyanobacterium Synechocystis sp. PCC 6803 show that the protein encoded by the psbJ gene regulates the number of photosystem II centers in thylakoid membranes.

The psbJ gene is a member of the psbEFLJ gene cluster in the cyanobacterium Synechocystis sp. PCC 6803 as well as in the chloroplasts of green plants. The putative product of the psbJ gene is a 4-kDa protein with one membrane-spanning domain. We have raised rabbit antibodies against a T7 gene 10-psbJ fusion protein, overexpressed in Escherichia coli. These antibodies recognized a polypeptide of expected size in the thylakoid membrane from wild type Synechocystis cells. We have also created a targeted mutant of Synechocystis 6803 in which the fourth codon of the psbJ open reading frame was modified to a translational stop codon. Thylakoid membranes from these mutant cells lacked the protein recognized by the antibodies. In the mutant cells, the partial electron transfer reaction mediated by the photosystem I complex was unaffected, whereas the rate of the photosystem II (PSII)-mediated reaction was 46% of that in wild type cells. Herbicide binding assays indicated that the PSII to chlorophyll ratio in the mutant cells was 49% of that in wild type cells. These results indicate that while the PsbJ protein is not essential for the photochemical activity it controls the amount of functionally assembled PSII complex in the thylakoid membrane.