Proton channel inactivation results in loss of chloroplast NDH complex activity

The physiological role of the plastidial photosynthetic complex I (formerly NAD(P)H dehydrogenase-like complex, NDH) within the electron transport chain of plants remains intriguing. While the NDH complex shares homology with complex I, a key component of the respiratory electron transport chain, electron transport rates through the NDH complex in thylakoids are relatively low. In this study, we used a structure-function approach and mutated the plastid genome-encoded ndhF gene to abolish the NdhF proton channel of the NDH complex. These mutations led to loss of plastoquinone reductase activity, indicating tight coupling between the proton and electron transfer reactions within NDH. Additionally, loss of the transverse helix of NdhF led to loss of the NDH complex, suggesting that this region of the NdhF subunit is required for complex stability. In agreement with previous studies using ndh knockout mutants, loss of NDH complex activity did not result in measurable changes in rates of steady-state cyclic electron flow. However, all mutants displayed a shift in the sensitivity of pH-dependent feedback regulation of the photosystem II antennae to total protonmotive force (pmf), indicating a possible defect in either stromal redox state or pmf distribution into ΔpH and Δψ.