Subunit Q Is Required To Stabilize The Large Complex Of NADPH Dehydrogenase In Synechocystis Sp. Strain PCC 6803

Cyanobacteria represent a remarkable group among prokaryotic microorganisms because of their ability to perform oxygenic photosynthesis. In addition to photosystem II, cytochrome b6 f, photosystem I and ATP synthase, in 1991, the fifth photosynthetic membrane protein complex, NADPH dehydrogenase(NDH-1), was discovered in cyanobacteria. Structurally, cyanobacterial NDH-1 complexes are similar to Escherichia coli and the mitochondrial respiratory chain NADPH oxidoreductase(complex I). Electron microscopy studies have shown that the NDH-1L complex is indeed L-shaped, with a relatively short hydrophilic arm. However, the homologous genes to three active subunits in catalytic domain of the E. coli complex I are absent in cyanobacterial genome. Functionally, cyanobacterial NDH-1 complexes participate in a variety of bioenergetic reactions, such as respiration, cyclic electron transport around photosystem I and CO2 uptake. Subsequent studies also demonstrate that NDH-1 complexes play important role in various physiological activities of cyanobacterial cells, even in their survival. Therefore, it is necessary to carry out a series of studies on the complexes.In cyanobacterial cells, the main routes of CET are NDH-1-mediated one(NDH-CET), which can greatly alleviate high light-sensitive growth phenotypes. Therefore, high light strategy can help in identifying the proteins essential to NDH-CET. Our important results are summarized as follows: firstly, two mutants sensitive to high light for growth and impaired in NDH-CET activity were successfully isolated from Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library by using high-light screening strategy. Further, both mutants were tagged in an open reading frame, which its product was highly homologous to Ndh Q. Secondly, we found that absence of Ndh Q destabilized the NDH-1L complex, but did not influence the accumulated levels of Ndh subu nits in thylakoid membrane. Further, Ndh Q is essential to stabilize the NDH-1L complex under thermal treatment. Consequently, absence of Ndh Q impaired respiration but notCO2 uptake. Finally, in the ndh P-deletion mutant(Δndh P) background, absence of Ndh Q almost completely disassembled the NDH-1L to NDH-1M, similar to the results observed in Δndh D1/D2 mutant. We therefore conclude that both Ndh Q and Ndh P are essential to stabilize the NDH-1L complex. In conclusion, absence of Ndh Q destabilizes the NDH-1L complex, thereby impairing the activities of NDH-CET and respiration generating a high light-sensitive growth phenotype.