NDH-1 is a key component of the cyclic-electron-transfer around photosystem I (PSI CET) pathway, an important antioxidant mechanism for efficient photosynthesis. Here, we report a 3.2-Å-resolution cryo-EM structure of the ferredoxin (Fd)-NDH-1L complex from the cyanobacterium Thermosynechococcus elongatus. The structure reveals three β-carotene and fifteen lipid molecules in the membrane arm of NDH-1L. Regulatory oxygenic photosynthesis-specific (OPS) subunits NdhV, NdhS and NdhO are close to the Fd-binding site whilst NdhL is adjacent to the plastoquinone (PQ) cavity, and they play different roles in PSI CET under high-light stress. NdhV assists in the binding of Fd to NDH-1L and accelerates PSI CET in response to short-term high-light exposure. In contrast, prolonged high-light irradiation switches on the expression and assembly of the NDH-1MS complex, which likely contains no NdhO to further accelerate PSI CET and reduce ROS production. We propose that this hierarchical mechanism is necessary for the survival of cyanobacteria in an aerobic environment.

Bioinspired by diatoms, biomimetic silicification confers an artificial shell on cyanobacteria to alleviate photoinhibition; thus, the photosynthesis of the resulting cyanobacteria@SiO₂ becomes more efficient under high light conditions.

Cyanobacterial NADPH:plastoquinone oxidoreductase, or type I NAD(P)H dehydrogenase, or the NDH-1 complex is involved in plastoquinone reduction and cyclic electron transfer (CET) around photosystem I. CET, in turn, produces extra ATP for cell metabolism particularly under stressful conditions. Despite significant achievements in the study of cyanobacterial NDH-1 complexes during the past few years, the entire subunit composition still remains elusive. To identify missing subunits, we screened a transposon-tagged library of Synechocystis 6803 cells grown under high light. Two NDH-1-mediated CET (NDH-CET)-defective mutants were tagged in the same ssl0352 gene encoding a short unknown protein. To clarify the function of Ssl0352, the ssl0352 deletion mutant and another mutant with Ssl0352 fused to yellow fluorescent protein (YFP) and the His₆ tag were constructed. Immunoblotting, mass spectrometry, and confocal microscopy analyses revealed that the Ssl0352 protein resides in the thylakoid membrane and associates with the NDH-1L and NDH-1M complexes. We conclude that Ssl0352 is a novel subunit of cyanobacterial NDH-1 complexes and designate it NdhS. Deletion of the ssl0352 gene considerably impaired the NDH-CET activity and also retarded cell growth under high light conditions, indicating that NdhS is essential for efficient operation of NDH-CET. However, the assembly of the NDH-1L and NDH-1M complexes and their content in the cells were not affected in the mutant. NdhS contains a Src homology 3-like domain and might be involved in interaction of the NDH-1 complex with an electron donor.

A novel NDH-1L-CpcG2-phycobilisome-PSI supercomplex essential to cyclic electron transport was identified in Synechocystis sp. strain PCC 6803.