Search

Thousands of nuclear-encoded proteins are transported into chloroplasts through the TOC–TIC translocon that spans the chloroplast envelope membranes. A motor complex pulls the translocated proteins out of the TOC–TIC complex into the chloroplast stroma by hydrolyzing ATP. The Orf2971–FtsHi complex has been suggested to serve as the ATP-hydrolyzing motor in Chlamydomonas reinhardtii , but little is known about its architecture and assembly. Here, we report the 3.2-Å resolution structure of the Chlamydomonas Orf2971–FtsHi complex. The 20-subunit complex spans the chloroplast inner envelope, with two bulky modules protruding into the intermembrane space and stromal matrix. Six subunits form a hetero-hexamer that potentially provides the pulling force through ATP hydrolysis. The remaining subunits, including potential enzymes/chaperones, likely facilitate the complex assembly and regulate its proper function. Taken together, our results provide the structural foundation for a mechanistic understanding of chloroplast protein translocation. The Orf2971–FtsHi complex in Chlamydomonas reinhardtii functions as an ATP-hydrolyzing motor for pulling proteins from the chloroplast translocon. Its 3.2-Å resolution structure reveals a 20-subunit complex spanning the chloroplast inner envelope with large modules in the intermembrane space and stromal matrix. Results suggest that the hetero-hexameric core generates pulling force and that other subunits assist in complex assembly and regulation, offering new insights into chloroplast protein translocation. [ABSTRACT FROM AUTHOR]

While photosynthesis transforms sunlight energy into sugar, aerobic and anaerobic respiration (fermentation) catabolizes sugars to fuel cellular activities. These processes take place within one cell across several compartments, however it remains largely unexplored how they interact with one another. Here we report that the weak acids produced during fermentation down-regulate both photosynthesis and aerobic respiration. This effect is mechanistically explained with an "ion trapping" model, in which the lipid bilayer selectively traps protons that effectively acidify subcellular compartments with smaller buffer capacities – such as the thylakoid lumen. Physiologically, we propose that under certain conditions, e.g., dim light at dawn, tuning down the photosynthetic light reaction could mitigate the pressure on its electron transport chains, while suppression of respiration could accelerate the net oxygen evolution, thus speeding up the recovery from hypoxia. Since we show that this effect is conserved across photosynthetic phyla, these results indicate that fermentation metabolites exert widespread feedback control over photosynthesis and aerobic respiration. This likely allows algae to better cope with changing environmental conditions. The processes of photosynthesis, aerobic and anaerobic respiration (fermentation) power life on Earth. Here, using mainly green alga Chlamydomonas, the authors find that the weak acids produced during fermentation could chemically suppress both photosynthesis and aerobic respiration. [ABSTRACT FROM AUTHOR]

Microalgae, as facultative aerobic organisms, convert solar energy to produce biohydrogen under anaerobic condition. Biohydrogen is a kind of ideal clean and renewable energy source with great commercial potential. Many endeavors have been focused on improving the biohydrogen yields by various means. Here, the research history of hydrogen production by microalgae (including cyanobacteria and green algae), the characteristics of nitrogenase and hydrogenase, the mechanisms of hydrogen production, the technological progress, and the application of enzymatic, genetic, and metabolic engineering methods to improve hydrogen production by microalgae are reviewed. In addition, the regulation of anaerobic metabolisms of Chlamydomonas reinhardtii after the disruption of key enzymes functioning in the fermentative pathways is discussed. Finally, the main challenges and obstacles facing the more efficient production and commercialization of hydrogen production from microalgae in the future are proposed. [ABSTRACT FROM AUTHOR]

Summary: Chromochloris zofingiensis, featured due to its capability to simultaneously synthesize triacylglycerol (TAG) and astaxanthin, is emerging as a leading candidate alga for production uses. To better understand the oleaginous mechanism of this alga, we conducted a multiomics analysis by systematically integrating time‐resolved transcriptomes, lipidomes and metabolomes in response to nitrogen deprivation. The data analysis unraveled the distinct mechanism of TAG accumulation, which involved coordinated stimulation of multiple biological processes including supply of energy and reductants, carbon reallocation from protein and starch, and 'pushing' and 'pulling' carbon to TAG synthesis. Unlike the model alga Chlamydomonas, de novo fatty acid synthesis in C. zofingiensis was promoted, together with enhanced turnover of both glycolipids and phospholipids, supporting the drastic need of acyls for TAG assembly. Moreover, genomewide analysis identified many key functional enzymes and transcription factors that had engineering potential for TAG modulation. Two genes encoding glycerol‐3‐phosphate acyltransferase (GPAT), the first committed enzyme for TAG assembly, were found in the C. zofingiensis genome; in vivo functional characterization revealed that extrachloroplastic GPAT instead of chloroplastic GPAT played a central role in TAG synthesis. These findings illuminate distinct oleaginousness mechanisms in C. zofingiensis and pave the way towards rational manipulation of this alga to becone an emerging model for trait improvements. Significance Statement: A multiomics study using systematically integrating time‐resolved transcriptomes, lipidomes and metabolomes was conducted for the emerging model alga C. zofingiensis. The 'omics data analysis, plus in vivo functional characterization of glycerol‐3‐phosphate acyltransferases, uncovered distinct oleaginousness characteristics and provided insights into rational manipulation of this alga for trait improvements. [ABSTRACT FROM AUTHOR]

Chlamydomonas reinhardtii insertion mutants disrupted for genes encoding acetate kinases (EC 2.7.2.1) (ACK1 and ACK2) and a phosphate acetyltransferase (EC 2.3.1.8) (PAT2, but not PAT1) were isolated to characterize fermentative acetate production. ACK1 and PAT2 were localized to chloroplasts, while ACK2 and PAT1 were shown to be in mitochondria. Characterization of the mutants showed that PAT2 and ACK1 activity in chloroplasts plays a dominant role (relative to ACK2 and PAT1 in mitochondria) in producing acetate under dark, anoxic conditions and, surprisingly, also suggested that Chlamydomonas has other pathways that generate acetate in the absence of ACK activity. We identified a number of proteins associated with alternative pathways for acetate production that are encoded on the Chlamydomonas genome. Furthermore, we observed that only modest alterations in the accumulation of fermentative products occurred in the ack1 , ack2 , and ack1 ack2 mutants, which contrasts with the substantial metabolite alterations described in strains devoid of other key fermentation enzymes. [ABSTRACT FROM AUTHOR]

Many microbes in the soil environment experience micro-oxic or anoxic conditions for much of the late afternoon and night, which inhibit or prevent respiratory metabolism. To sustain the production of energy and maintain vital cellular processes during the night, organisms have developed numerous pathways for fermentative metabolism. This review discusses fermentation pathways identified for the soil-dwelling model alga Chlamydomonas reinhardtii, its ability to produce molecular hydrogen under anoxic conditions through the activity of hydrogenases, and the molecular flexibility associated with fermentative metabolism that has only recently been revealed through the analysis of specific mutant strains. [ABSTRACT FROM AUTHOR]

Abstract: Chlamydomonas reinhardtii (Chlamydomonas throughout) encodes two [FeFe]-hydrogenases, designated HYDA1 and HYDA2. While HYDA1 is considered the dominant hydrogenase, the role of HYDA2 is unclear. To study the individual functions of each hydrogenase and provide a platform for future bioengineering, we isolated the Chlamydomonas hydA1-1, hydA2-1 single mutants and the hydA1-1 hydA2-1 double mutant. A reverse genetic screen was used to identify a mutant with an insertion in HYDA2, followed by mutagenesis of the hydA2-1 strain coupled with a H2 chemosensor phenotypic screen to isolate the hydA1-1 hydA2-1 mutant. Genetic crosses of the hydA1-1 hydA2-1 mutant to wild-type cells allowed us to also isolate the single hydA1-1 mutant. Fermentative, photosynthetic, and in vitro hydrogenase activities were assayed in each of the mutant genotypes. Surprisingly, analyses of the hydA1-1 and hydA2-1 single mutants, as well as the HYDA1 and HYDA2 rescued hydA1-1 hydA2-1 mutant demonstrated that both hydrogenases are able to catalyze H2 production from either fermentative or photosynthetic pathways. The physiology of both mutant and complemented strains indicate that the contribution of HYDA2 to H2 photoproduction is approximately 25% that of HYDA1, which corresponds to similarly low levels of in vitro hydrogenase activity measured in the hydA1-1 mutant. Interestingly, enhanced in vitro and fermentative H2 production activities were observed in the hydA1-1 hydA2-1 strain complemented with HYDA1, while maximal H2-photoproduction rates did not exceed those of wild-type cells. [Copyright &y& Elsevier]