TARGETED DELETION OF PSAJ FROM THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803 INDICATES STRUCTURAL INTERACTIONS BETWEEN THE PSAJ AND PSAF SUBUNITS OF PHOTOSYSTEM-I

Photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c(6) and the reduction of ferredoxin or flavodoxin. PsaJ is a 4.4 kDa hydrophobic subunit of photosystem I from cyanobacteria and chloroplasts. To investigate the function of PsaJ, we generated a mut ant strain of the cyanobacterium Synechocystis sp. PCC 6803 in which t he psaJ gene is replaced by a gene for chloramphenicol resistance. Del etion of psaJ led to a reduction in the steady state RNA level from ps aF which is located upstream from psaJ. Immunoquantification using an anti-PsaF antibody revealed a significant decrease in the amount of Ps aF in membranes of the mutant strain. Trimeric photosystem I complexes isolated from the mutant strain using n-dodecyl beta-D-maltoside lack ed PsaJ, contained ca. 80% less PsaF, but maintained wild-type levels of other photosystem I subunits. In contrast, the photosystem I purifi ed using Triton X-100 contained less than 2% PsaF when compared to the wild type, showing the more extractable nature of PsaF in PsaJ-less p hotosystem I in the presence of Triton X-100. PsaE was more accessible to removal by NaI in a mutant strain lacking PsaF and PsaJ than in th e wild type. The presence of PsaF in photosystem I from the PsaJ-less strain did not alter the increased susceptibility of PsaE to removal b y NaI. These results indicate an interaction between PsaJ and PsaF in the organization of the complex.