AN INDEPENDENT ROLE OF CYTOCHROME C-550 IN CYANOBACTERIAL PHOTOSYSTEM-II AS REVEALED BY DOUBLE-DELETION MUTAGENESIS OF THE PSBO AND PSBV GENES IN SYNECHOCYSTIS SP PCC-6803

Cytochrome (cyt) c-550 and the 33 kDa protein are two extrinsic compon ents that function in maintaining oxygen evolution in cyanobacterial c ells. Deletion of either of the two components has been shown to resul t in cyanobacterial phenotypes that are still capable of photoautotrop hic growth albeit with a reduced rate. In order to study the function of cyt c-550 in cyanobacterial photosystem II (PSII) and its possible interaction with the 33 kDa extrinsic protein, we constructed a mutant lacking both cyt c-550 and the 33 kDa protein by inactivating the psb V and psbO genes simultaneously in a cyanobacterium, Synechocystis sp. PCC 6803. The resultant double-deletion mutant was unable to grow pho toautotrophically and showed almost no oxygen-evolving activity (less than 10% of the wild type). This residual activity was also lost rapid ly upon illumination, suggesting an increased sensitivity of the mutan t cells toward photoinhibition. Thermoluminescence measurements indica ted that the mutant virtually cannot undergo normal charge accumulatio n (S-state transitions) leading to oxygen evolution. Herbicide-binding and Western blot analyses showed that the mutant accumulates the PSII complex to an extent of only 20% of that in wild-type cells. Combined with previous results, the present results indicated that cyt c-550 s upported oxygen evolution in the single-deletion mutant lacking the 33 kDa protein alone and vice versa. Thus, both cyt c-550 and the 33 kDa protein function independently in maintaining cyanobacterial oxygen-e volving activity in vivo, and both of them are required for the optima l activity. This not only provides another conclusive Line of evidence for the concept that cyt c-550 functions in cyanobacterial PSII oxyge n evolution but also reveals a functional difference between this extr insic cyt in cyanobacterial PSII and the extrinsic 23 kDa protein in h igher plant PSII, as the 23 kDa protein alone cannot support oxygen ev olution in the absence of the 33 kDa protein.