Construction of cyanobacterial mutant strains which lack a specific subunit of photo system I and the characterization of their photosynthetic properties.

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Photosystem I (PSI) is a multisubunit pigment-protein complex that consists of more than 10 different polypeptides (from PsaA to PsaM) and catalyzes light-driven electron transfer from cytochrome c_6/plastocyanin to ferredoxin. In this study, we have generated the first PsaK-deficient mutant strains of Synechocystis sp.PCC 6803 byogene-targeting in order to identify roles of PsaK in PSI function and organization. Nothing is known about the function of PsaK.DNA fragment containing psaK was amplified from the Synechocystis sp.PCC 6803 genomic DNA by PCR with using nucleotide primers made according the sequence information for psaK which was obtained from Cyanobase, the database on the genome of Synechocystis sp.PCC 6803. The PCR fragment was then cloned into pGEM-T vector. A mutant strain lacking PsaK was generated by transforming the wild type cells with cloned DNA in which psaK gene was interrupted by a gene conferring resistance to kanamycin. Northern blot analysis and SDS polyacrylamide gel electrophoresis revealed that the mutant strain lacked both the transcript and translation products of psaK.The amount of chlorophyll and phycobilisome per cell basis was reduced, suggesting the involvement of PsaK with the binding of those pigments. However, photoautotrophic growth and photosynthetic characteristics of the mutant strain were similar to those of the wild type.The mutational studies have shown that mutants lacking one (or two) of the accessory subunits of PSI photosynthesize as actively as the wild type. The accessory subunits of PSI do not bind any redox-active cnfactors like PsaA,PsaB and PsaC.In order to find physiological importance of these accessory subunit, we compared phenotypes of the mutant strains with that of the wild type under various stress conditions. We have obtained some evidences which demonstrate that PsaD and PsaE are important for the protection of cyanobacterial cells from photoinhibition and heat-damage.