Fig. 2: Disruption of a cyanobacteria-derived intermolecular salt bridge impairs NDH-CET activity.

a Schematic model of electron flow from reduced ferredoxin (Fd) by photosystem I (PSI) to plastoquinone (PQ) via photosynthetic NDH-1 in this assay. Synechocystis 6803 cells were cultured under a light intensity of 40 µmol photons m⁻² s⁻¹ with 2% CO₂ in air and harvested during the logarithmic growth phase for experiments (b–e). b In vitro analysis of NDH-CET activity. Increases in chlorophyll fluorescence by addition of Fd under the illumination of actinic light (AL; 620 nm, 918 µmol photons m⁻² s⁻¹) were monitored in thylakoid membranes (80 µg chlorophyll a mL⁻¹) of the wild type (WT) Synechocystis 6803 and its mutants. c–e In vivo analysis of NDH-CET activity. c Monitoring of NDH-CET activity using chlorophyll fluorescence analysis. The top curve shows a typical trace of chlorophyll fluorescence in the WT Synechocystis 6803. Cells were exposed to AL (620 nm; 45 µmol photons m⁻² s⁻¹) for 30 s. AL was turned off, and the subsequent change in the chlorophyll fluorescence level was monitored as an indication of NDH-CET activity. d Redox kinetics of P700 after termination of AL illumination (800 µmol photons m⁻² s⁻¹) under a background of far-red (FR) light. The cells were illuminated by AL supplemented with FR light to store electrons in the cytoplasmic pool. After termination of AL illumination, P700⁺ was transiently reduced by electrons from the PQ pool; thereafter, P700 was reoxidized by background FR light. The redox kinetics of P700 were recorded. The P700⁺ levels were standardized by their maximum levels attained by exposure to FR light. e Kinetics of the P700⁺ rereduction in darkness after turning off FR light in the presence of 10 µM 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). The chlorophyll a concentration was adjusted to 20 µg mL⁻¹ before measurement, and curves are normalized to the maximal signal.