| Summary: | 博士 === 國防醫學院 === 生命科學研究所 === 92 === Synechococcus sp. RF-1, a unicellular N2-fixing cyanobacterium, can grow photosynthetically and diazotrophically in continuous light. How the organism protects its nitrogenase from damage by oxygen is unclear. In this work, a PAM-101 fluorometer and an O2 electrode are used simultaneously to study the chlorophyll a fluorescence and to monitor O2 exchanges in Synechococcus sp. RF-1 cells. The results found that in addition to CO2, O2 can also act as electron acceptor to receive electrons derived from QA-, i.e. O2 photoreduction. Studies with various inhibitors of the electron transport chain demonstrated that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6- isopropyl-p-benzo-quinone (DBMIB) and salicylhydroxamic acid (SHAM) inhibited the photoreduction of O2, while glycolaldehyde, disalicylidenepropanediamine (DSPD), methyl viologen (MV) and KCN did not. Addition of N, N, N’, N’-tetramethyl-p-phenylenediamine (TMPD) bypassed the inhibition of DBMIB. The soluble proteins extracted from Synechococcus sp. RF-1 cells revealed cytochrome c-dependent, KCN-resistant and SHAM-sensitive O2 uptake activities. These results imply that a KCN-resistant and SHAM-sensitive oxidase transfers electrons generated from photosystem II to O2 between cytochrome b6f complex and photosystem I. The oxidase can use reduced cytochrome c as substrate and results in the photoreduction of O2. Besides Synechococcus sp. RF-1, we used the same system to detect the chlorophyll a fluorescence of other cyanobacteria. Studies with non-nitorgen fixation species revealed no O2 photoreduction acticity in Synechococcus PCC 7942 and only low activity in Synechocystis PCC 6803, while nitrogen-fixing species, Cyanothece ATCC 51142 and Gloeothece sp. displayed high activities. The results suggest that photoreduction of O2 may be associated with protection of nitrogenase. In Synechococcus sp. RF-1 cells, which use a KCN-resistant and SHAM-sensitive oxidase to perform this function. When SHAM blocked this alternative electron transport pathway, it decreased 83 % of dinitrogen-fixing activity in continuous light growing cells, but only decreased 38 % of the activity in 12 light/12 dark regimen-growing cells. We conclude that a KCN-resistant and SHAM-sensitive oxidase dependent O2 photoreduction is an important mechanism to reduce the oxidative stress of photosystem and to protect the nitrogenase from the damage of oxygen for Synechococcus sp. RF-1 cells.
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