Photosynthetic Electron Transport Involved in PxcA-Dependent Proton Extrusion in Synechocystis sp. Strain PCC6803: Effect of pxcA Inactivation on CO 2 , HCO 3 − , and NO 3 − Uptake

Abstract

ABSTRACT 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 CO 2 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 CO 2 fixation is inhibited. At pH 8.0, NO 3 − 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 CO 2 or NO 3 − at low [Na + ] and showed very low CO 2 uptake activity even at 15 mM Na + . A possible role of PxcA-dependent proton exchange in charge and pH homeostasis during uptake of CO 2 , HCO 3 − , and NO 3 − is discussed.