Photosynthetic electron transport involved in PxcA-dependent proton extrusion in Synechocystis sp. strain PCC6803: Effect of pxcA inactivation on CO₂, HCO₃^-, and NO₃^- uptake

The product of pxcA (formerly known as cotA) is involved in light- induced Na⁺-dependent proton extrusion. In the presence of 2,5-dimethyl-p- benzoquinone, net proton extrusion by Synechocystis sp. strain PCC6803 ceased after 1 min of illumination and a postillumination influx of protons was observed, suggesting that the PxcA-dependent, light-dependent proton extrusion equilibrates with a light-independent influx of protons. A photosystem I (PS I) deletion mutant extruded a large number of protons in the light. Thus, PS II-dependent electron transfer and proton translocation are major factors in light-driven proton extrusion, presumably mediated by ATP synthesis. Inhibition of CO₂ fixation by glyceraldehyde in a cytochrome c oxidase (COX) deletion mutant strongly inhibited the proton extrusion. Leakage of PS II-generated electrons to oxygen via COX appears to be required for proton extrusion when CO₂ fixation is inhibited. At pH 8.0, NO₃^- uptake activity was very low in the pxcA mutant at low [Na⁺] (~100 μM). At pH 6.5, the pxcA strain did not take up CO₂ or NO₃^- at low [Na⁺] and showed very low CO₂ uptake activity even at 15 mM Na⁺. A possible role of PxcA-dependent proton exchange in charge and pH homeostasis during uptake of CO₂, HCO₃^-, and NO₃^- is discussed.