| It had been known that there are mainly four kinds of complexes, photosystem II (PSII),cytochrome b6/f, photosystem I (PSI) and ATP synthase, in cyanobacterial and higher plantcloroplastic thylakoid membranes. Since mid-1980's, another protein complex, NAD(P)Hdehydrogenase (NDH) complex, with a high degree homology to mitochondrial complex I hasbeen identified in thylakoid membranes of cyanobacteria and higher plants. NDH complex hasbeen suggested to function as a mediator of both PSI-cyclic and respiratory electron flow tothe intersystem chain in cyanobacteria or of PSI-cyclic and chlororespiratory electron flow inhigher plants. Evidences show that NDH complex has very important physiological functionsin cyanobacteria and is even indispensable to the viability of cyanobacteria. Therefore,presently, more and more attentions are being paid to this area. Up to date, studies on thestructure and functions of NDH complex are still in primary stage. This work mainly focuseson the roles of NDH complex in the responses and acclimation of cyanobacteria to low CO2concentration, the separation and purification of NDH complex and its interaction withphycobiliproteins.1. Effects of low CO2 on cyanobacterial NDH complex The expression and activity of type-1 NAD(P)H dehydrogenase (NDH) was comparedbetween cells of Synechocystis PCC6803 grown in high (H-cells) and low (L-cells) CO2 conditions.Western analysis indicated that L-cells contain higher amounts of the NDH subunits, NdhH, NdhIand NdhK. An NADPH-specific subcomplex of NDH showed higher NADPH-nitroblue tetrazoliumoxidoreductase activity in L-cells. The activities of both NADPH-menadione oxidoreductase and VIIlight-dependent NADPH oxidation driven by photosystem I were much higher in L-cells than inH-cells. The initial rate of re-reduction of P700+ following actinic light illumination in the presence ofDCMU under background far-red light was enhanced in L-cells. In addition, rotenone, a specificinhibitor of NDH, suppressed the relative rate of post-illumination increase in Chl fluorescence ofL-cells more than that of H-cells, suggesting that the involvement of NDH in cyclic electron flowaround photosystem I was enhanced by low CO2. Taken together, these results suggest that NDHcomplex and NDH mediated-cyclic electron transport are stimulated by low CO2 and function in theacclimation of cyanobacteria to low CO2.2. Response of NDH complex to the alteration of CO2 concentration An NADPH-specific NDH subcomplex was separated by native-polyacrylamide gelelectrophoresis and detected by activity staining from the whole cell extracts of SynechocystisPCC6803. Low CO2 caused increase in the activity of this subcomplex quickly, accompanied by anevident increase in the expression of NdhK and PSI-driven NADPH oxidation activity that canreflect the activity of NDH-mediated cyclic electron transport. During incubation with high CO2,the activities of NDH subcomplex and PSI-driven NADPH oxidation as well as the proteinlevel of NdhK slightly increased at the beginning, but decreased evidently in various degreesalong with incubation time. These results suggest that CO2 concentration in vitro as a signal cancontrol the activity of NDH complex and NDH complex may in turn function in the regulation of CO2uptake3. Separation of hydrophobic NDH subcomplexes from Synechocystis PCC6803 Some efforts have been performed to separate an integrated NA(D)PH dehydrogenase(NDH) complex from cyanobacteria, which proved the purification of NDH complex verydifficult due to its friability in vitro. Until now, no report has ever been showed to illustrate thepurification of a hydrophobic NDH subcomplex from cyanobacteria yet. In this work, two NDHsubcomplexes were separated from n-dodecyl β-D-maltoside (DM)-treated whole cell extractsof Synechocystis PCC6803 by anion exchange chromatography and gel filtration. Bothsubcomplexes contained the hydrophobic subu...
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