FUNCTION AND ORGANIZATION OF PHOTOSYSTEM-I IN A CYANOBACTERIAL MUTANTSTRAIN THAT LACKS PSAF AND PSAJ SUBUNITS

Photosystem I functions as a light-driven plastocyanin-ferredoxin oxid oreductase in the photosynthetic membranes of cyanobacteria and chloro plasts. A mutant strain of the cyanobacterium Synechocystis sp. PCC 68 03 that contains a deletion of the psaF gene and a transcriptionally i nactive psaJ gene has assembled photosystem I complexes that lack PsaF , a lumenal protein and PsaJ, a 4-kDa hydrophobic protein. The cells o f the mutant and wild type strains have similar rates of photosyntheti c electron transfer and P700+ rereduction under linear and cyclic elec tron transfer conditions. Analysis of flash-induced absorption transie nts -at 700 nm demonstrate that the absence of PsaF in purified mutant photosystem I did not affect the rate of P700 rereduction by cytochro me c553. Therefore, PsaF is not essential for docking of cytochrome c5 53. We also studied the organization of the proteins of mutant and wil d type photosystem I by comparing their accessibility to digestion by thermolysin or to removal by 1 m NaI. The PsaA-PsaB subunits were more easily degraded by thermolysin in the mutant photosystem I. Thermolys in cleavage of PsaB yielded two major fragments that were immunoreacti ve with an antibody raised against the C terminus of PsaB. The N termi ni of these PsaB peptides mapped at Ile482 and Ile498 residues, thus i dentifying a surface-exposed domain of the core of photosystem I. The PsaE subunit could be removed by 1 m NaI and was rapidly digested by t hermolysin in the mutant but not in the wild type photosystem I. There fore, PsaF and PsaJ subunits of photosystem I have dispensable accesso ry roles in the function and organization of the complex.