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 photosynthesisspecific (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 highlight stress. NdhV assists in the binding of Fd to NDH-1L and accelerates PSI CET in response to short-term highlight exposure. In contrast, prolonged highlight 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.

Two mutants isolated from a tagging library of Synechocystis sp. strain PCC 6803 were sensitive to high light and had a tag in sll1471 encoding CpcG2, a linker protein for photosystem I (PSI)-specific antenna. Both mutants demonstrated strongly impaired NDH-1-dependent cyclic electron transport. Blue native-polyacrylamide gel electrophoresis followed by immunoblotting and mass spectrometry analyses of the wild type and a mutant containing CpcG2 fused with yellow fluorescent protein-histidine6 indicated the presence of a novel NDH-1L-CpcG2-PSI supercomplex, which was absent in the cpcG2 deletion mutant, the PSI-less mutant, and several other strains deficient in NDH-1L and/or NDH-1M. Coimmunoprecipitation and pull-down analyses on CpcG2-yellow fluorescent protein-histidine6, using antibody against green fluorescent protein and nickel column chromatography, confirmed the association of CpcG2 with the supercomplex. Conversely, the use of antibodies against NdhH or NdhK after blue native-polyacrylamide gel electrophoresis and in coimmunoprecipitation experiments verified the necessity of CpcG2 in stabilizing the supercomplex. Furthermore, deletion of CpcG2 destabilized NDH-1L as well as its degradation product NDH-1M and significantly decreased the number of functional PSI centers, consistent with the involvement of CpcG2 in NDH-1-dependent cyclic electron transport. The CpcG2 deletion, however, had no effect on respiration. Thus, we propose that the formation of an NDH-1L-CpcG2-PSI supercomplex in cyanobacteria facilitates PSI cyclic electron transport via NDH-1L.

Methylation of cytosine to 5-methylcytosine (5mC) is a prevalent DNA modification found in many organisms. Sequential oxidation of 5mC by TET dioxygenases results in a cascade of additional epigenetic marks and promotes DNA demethylation in mammals1,2. However, the enzymatic activity and the function of TET homologs in diverse eukaryotes remains largely unexplored. In our study of TET homologs in the green alga Chlamydomonas reinhardtii, we have found a 5mC-modifying enzyme (CMD1) that catalyzes conjugation of a glyceryl moiety to the methyl group of 5mC through a carbon-carbon bond, resulting in two novel stereoisomeric nucleobase products. The catalytic activity of CMD1 requires Fe(II) and the integrity of its binding motif His-x-Asp (HxD), which is conserved in Fe-dependent dioxygenases3. However, unlike all previous described TET enzymes which utilize 2-oxoglutarate (2-OG) as a co-substrate4, CMD1 utilizes L-ascorbic acid (vitamin C, VC) as an essential co-substrate. VC donates the glyceryl moiety to 5mC with concurrent formation of glyoxylic acid and CO2. The VC-derived DNA modification is present in the genome of C. reinhardtii and its level decreases significantly in a CMD1 mutant strain. The fitness of CMD1 mutant cells during high light exposure is reduced. LHCSR3, a critical gene for protection of C. reinhardtii from photooxidative damage in high light, is hypermethylated and downregulated compared to wild-type cells, causing a lowered capacity for photoprotective non-photochemical quenching (NPQ). Our study thus reveals a new eukaryotic DNA base modification, which is catalyzed by a divergent TET homolog and unexpectedly derived from VC, and its role as a potential epigenetic mark that may counteract DNA methylation in the regulation of photosynthesis.

The accelerated thermo-oxidative aging of acrylonitrile–butadiene rubber (NBR) was studied at elevated temperatures. The chemical structure characterized by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and pyrolysis gas chromatography–mass spectroscopy (PGC-MS) showed the loss of low-molecular-weight additives, such as antioxidants and paraffin, and the formation of carbonyl groups and unsaturated double bonds. The cross-linking degree characterized by NMR and a swelling test showed that aging is a competitive process of cross-linking and chain scission. Cross-linking dominated the thermal aging of NBR most of the time, whereas chain scission began to increase after a long time at high temperatures. The changes of mechanical property magnitudes during thermal aging of NBR were studied by using the recovery from bending test (RFB) and tensile test. By comparing the physical property results and the structural changes, their relationship is discussed.

Background: There is no report on Ndh subunits that destabilize the NDH-1 complex and repress activity. Results: Deletion of ndhO in Synechocystis 6803 increased the activity of cyclic electron transport around photosystem I, whereas overexpression repressed the activity and destabilized NDH-1M complex. Conclusion: NdhO destabilizes NDH-1M and represses the activity. Significance: NdhO is a new type subunit that controls NDH-1M negatively.