ABSENCE OF PSAC SUBUNIT ALLOWS ASSEMBLY OF PHOTOSYSTEM-I CORE BUT PREVENTS THE BINDING OF PSAD AND PSAE IN SYNECHOCYSTIS SP PCC6803

In photosystem I (PSI) of oxygenic photosynthetic organisms the psaC p olypeptide, encoded by the psaC gene, provides the ligands for two [4F e-4S] clusters, F-A and F-B. Unlike other cyanobacteria, two different psaC genes have been reported in the cyanobacterium Synechocystis 680 3, one (copy 1) with a deduced amino acid sequence identical to that o f tobacco and another (copy 2) with a deduced amino acid sequence simi lar to those reported for other cyanobacteria. Insertion of a gene enc oding kanamycin resistance into copy 2 resulted in a photosynthesis-de ficient strain, CDK25, lacking the PsaC, PsaD and PsaE polypeptides in isolated thylakoid membranes, while the PsaA/PsaB and PsaF subunits w ere found. Growth of the mutant cells was indistinguishable from that of wild-type cells under light-activated heterotrophic growth (LAHG). A reversible P700(+) signal was detected by EPR spectroscopy in the is olated thylakoids during illumination at low temperature. Under these conditions, the EPR signals attributed to F-A and F-B were absent in t he mutant strain, but a reversible F-x signal was present with broad r esonances at g = 2.079, 1.903, and 1.784. Addition of PsaC and PsaD pr oteins to the thylakoids gave rise to resonances at g = 2.046, 1.936, 1.922, and 1.880; these values are characteristic of an interaction-ty pe spectrum of F-A(-) and F-B(-). In room-temperature optical spectros copic analysis, addition of PsaC and PsaD to the thylakoids also resto red a 30 ms kinetic transient which is characteristic of the P700(+)[F -A/F-B](-) backreaction. Expression of copy 1 was not detected in cell s grown under LAHG and under mixotrophic conditions. These results dem onstrate that copy 2 encodes the PsaC polypeptide in PSI in Synechocys tis 6803, while copy 1 is not involved in PSI; that the PsaC polypepti de is necessary for stable assembly of PsaD and PsaE into PSI complex in vivo; and that PsaC, PsaD and PsaE are not needed for assembly of P saA-PsaB dimer and electron transport from P700 to F-x.