Targeted inactivation of the psaK1, psaK2 and psaM genes encoding subunitsof Photosystem I in the cyanobacterium Synechocystis sp PCC 6803

PsaK and PsaM are small, integral membrane proteins, which are associated w ith the Photosystem I complexes of cyanobacteria. The complete genome seque nce of Synechocystis sp. PCC 6803 has revealed the presence of two unlinked psaK genes: psaK1 (ssr0390) and psaK2 (sll0629). To investigate structural and functional roles of the PsaK1, PsaK2 and PsaM polypeptides in Synechoc ystis sp. PCC 6803, we generated targeted mutants that lack the functional psaK1, psaK2 or psaM genes. Inactivation of psaK1, psaK2 or psaM did not af fect photoautotrophic growth, photosynthetic activity and accumulation of o ther subunits of the Photosystem I complex. The psaK1(-), psaK2(-) and psaK 1(-) psaK2(-) mutants showed normal levels of Photosystem I trimers, wherea s the lack of PsaM resulted in a 75% reduction in the recovery of trimers c ompared to the wild type. A 6.2 kDa polypeptide was observed in the Photosy stem I preparations from the wild type, but not from the psaK2(-) strain, s uggesting the presence of PsaK2 in the Photosystem I complexes. Using rever se-transcription and polymerase chain reaction, we confirmed the expression of the psaK2 gene in Synechocystis sp. PCC 6803. To conclude, both psaK1 a nd psaK2 are expressed in Synechocystis sp. PCC 6803 and the absence of bot h proteins results in only a small reduction in Photosystem I electron tran sport. The PsaM subunit is required for the formation of stable Photosystem I trimers.