Cytochrome (cyt) c-550 and the 33 kDa protein are two extrinsic components that function in maintaining oxygen evolution in cyanobacterial cells. Deletion of either of the two components has been shown to result in cyanobacterial phenotypes that are still capable of photoautotrophic 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 psbV and psbO genes simultaneously in a cyanobacterium, Synechocystis sp. PCC 6803. The resultant double-deletion mutant was unable to grow photoautotrophically and showed almost no oxygen-evolving activity (less than 10% of the wild type). This residual activity was also lost rapidly upon illumination, suggesting an increased sensitivity of the mutant cells toward photoinhibition. Thermoluminescence measurements indicated that the mutant virtually cannot undergo normal charge accumulation (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 supported 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 oxygenevolving activity in vivo, and both of them are required for the optimal activity. This not only provides another conclusive line of evidence for the concept that cyt c-550 functions in cyanobacterial PSII oxygen evolution but also reveals a functional difference between this extrinsic cyt in cyanobacterial PSII and the extrinsic 23 kDa protein in higher plant PSII, as thte-23 kDa protein alone cannot support oxygen evolution in the absence of the 33 kDa protein.
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