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  1. Abstract

    Mitochondrial cytochromecmaturation (CCM) requires heme attachment via distinct pathways termed systems I and III. The mosaic distribution of these systems in Archaeplastida raises questions about the genetic mechanisms and evolutionary forces promoting repeated evolution. Here, we show a recurrent shift from ancestral system I to the eukaryotic-specific holocytochromecsynthase (HCCS) of system III in 11 archaeplastid lineages. Archaeplastid HCCS is sufficient to rescue mutants of yeast system III and Arabidopsis system I. Algal HCCS mutants exhibit impaired growth and respiration, and altered biochemical and metabolic profiles, likely resulting from deficient CCM and reduced cytochromec-dependent respiratory activity. Our findings demonstrate that archaeplastid HCCS homologs function as system III components in the absence of system I. These results elucidate the evolutionary trajectory and functional divergence of CCM pathways in Archaeplastida, providing insight into the causes, mechanisms, and consequences of repeated cooption of an entire biological pathway.

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  2. Summary

    Understanding the unique features of algal metabolism may be necessary to realize the full potential of algae as feedstock for the production of biofuels and biomaterials. Under nitrogen deprivation, the green algaC. reinhardtiishowed substantial triacylglycerol (TAG) accumulation and up‐regulation of a gene,GPD2, encoding a multidomain enzyme with a putative phosphoserine phosphatase (PSP) motif fused to glycerol‐3‐phosphate dehydrogenase (GPD) domains. CanonicalGPDenzymes catalyze the synthesis of glycerol‐3‐phosphate (G3P) by reduction of dihydroxyacetone phosphate (DHAP). G3P forms the backbone ofTAGs and membrane glycerolipids and it can be dephosphorylated to yield glycerol, an osmotic stabilizer and compatible solute under hypertonic stress. RecombinantChlamydomonasGPD2 showed both reductase and phosphatase activitiesin vitroand it can work as a bifunctional enzyme capable of synthesizing glycerol directly fromDHAP. In addition,GPD2and a gene encoding glycerol kinase were up‐regulated inChlamydomonascells exposed to high salinity.RNA‐mediated silencing ofGPD2revealed that the multidomain enzyme was required forTAGaccumulation under nitrogen deprivation and for glycerol synthesis under high salinity. Moreover, aGPD2‐mCherry fusion protein was found to localize to the chloroplast, supporting the existence of aGPD2‐dependent plastid pathway for the rapid synthesis of glycerol in response to hyperosmotic stress. We hypothesize that the reductase and phosphatase activities ofPSPGPDmultidomain enzymes may be modulated by post‐translational modifications/mechanisms, allowing them to synthesize primarily G3P or glycerol depending on environmental conditions and/or metabolic demands in algal species of the core Chlorophytes.

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