Impaired Mitochondrial Transcription Termination Disrupts the Stromal Redox Poise in Chlamydomonas

In photosynthetic eukaryotes, the metabolite exchange between chloroplast and mitochondria ensures efficient photosynthesis under saturating light conditions. The Chlamydomonas reinhardtii mutant stm6 is devoid of the mitochondrial transcription termination factor MOC1 and aberrantly expresses the mitochondrial genome, resulting in enhanced photosynthetic hydrogen production and diminished light tolerance. We analyzed the modulation of mitochondrial and chlororespiration during the acclimation of stm6 and the MOC1-complemented strain to excess light. Although light stress stimulated mitochondrial respiration via the energy-conserving cytochrome c pathway in both strains, the mutant was unable to fine-tune the expression and activity of oxidative phosphorylation complex I in excess light, which was accompanied by an increased mitochondrial respiration via the alternative oxidase pathway. Furthermore, stm6 failed to fully activate chlororespiration and cyclic electron flow due to a more oxidized state of the chloroplast stroma, which is caused by an increased mitochondrial electron sink capacity. Increased susceptibility to photoinhibition of PSII in stm6 demonstrates that the MOC1-dependent modulation of mitochondrial respiration helps control the stromal redox poise as a crucial part of high-light acclimation in C. reinhardtii

Impaired Mitochondrial Transcription Termination Disrupts the Stromal Redox Poise in Chlamydomonas.

Uhmeyer A, Cecchin M, Ballottari M, Wobbe L. Impaired Mitochondrial Transcription Termination Disrupts the Stromal Redox Poise in Chlamydomonas. Plant Physiol. 2017;174(3):1399-1419

Functional analysis of three type-2 DGAT homologue genes for triacylglycerol production in the green microalga Chlamydomonas reinhardtii

La Russa M, Bogen C, Uhmeyer A, et al. Functional analysis of three type-2 DGAT homologue genes for triacylglycerol production in the green microalga Chlamydomonas reinhardtii. Journal of Biotechnology. 2012;162(1):13-20.Photosynthetic organisms like plants and algae can use sunlight to produce lipids as important metabolic compounds. Plant-derived triacylglycerols (TAGs) are valuable for human and animal nutrition because of their high energy content and are becoming increasingly important for the production of renewable biofuels. Acyl-CoA:diacylglycerol acyltransferases (DGATs) have been demonstrated to play an important role in the accumulation of TAG compounds in higher plants. DGAT homologue genes have been identified in the genome of the green alga Chlamydomonas reinhardtii, however their function in vivo is still unknown. In this work, the three most promising type-2 DGAT candidate genes potentially involved in TAG lipid accumulation (CrDGAT2a, b and c) were investigated by constructing overexpression strains. For each of the genes, three strains were identified which showed enhanced mRNA levels of between 1.7 and 29.1 times that of the wild type (wt). Total lipid contents, neutral lipids and fatty acid profiles were determined and showed that an enhanced mRNA expression level of the investigated DGAT genes did not boost the intracellular TAG accumulation or resulted in alterations of the fatty acid profiles compared to wild type during standard growth condition or during nitrogen or sulfur stress conditions. We conclude that biotechnological efforts to enhance cellular TAG amount in microalgae need further insights into the complex network of lipid biosynthesis to identify potential bottlenecks of neutral lipid production