Responsibility of regulatory gene expression and repressed protein synthesis for triacylglycerol accumulation on sulfur-starvation in Chlamydomonas reinhardtii

Abstract

Triacylglycerol (TG) synthesis is induced for energy and carbon storage in algal cells under nitrogen(N)-starved conditions, and helps prevent reactive oxygen species (ROS) production through fatty acid synthesis that consumes excessive reducing power. Here, the regulatory mechanism for the TG content in sulfur(S)-starved cells of Chlamydomonas reinhardtii was examined, in comparison to that in N- or phosphorus(P)-starved cells. S- and N- starved cells exhibited markedly increased TG contents with up-regulation of mRNA levels of diacylglycerol acyltransferase (DGAT) genes. S-Starvation also induced expression of the genes for phosphatidate synthesis. In contrast, P-starved cells exhibited little alteration of the TG content with almost no induction of these genes. The results implied deficient nutrient-specific regulation of the TG content. An arg9 disruptant defective in arginine synthesis, even without nutritional deficiencies, exhibited an increased TG content upon removal of supplemented arginine, which repressed protein synthesis. Repression of protein synthesis thus seemed crucial for TG accumulation in S- or N- starved cells. Meanwhile, the results of inhibitor experiments involving cells inferred that TG accumulation during S-starvation is supported by photosynthesis and de novo fatty acid synthesis. During S-starvation, sac1 and snrk2.2 disruptants, which are defective in the response to the ambient S-status, accumulated TG at lower and higher levels, respectively, than the wild type. The sac1 and snrk2.2 disruptants showed no or much greater up-regulation of DGAT genes, respectively. In conclusion, TG synthesis would be activated in S-starved cells, through the diversion of metabolic carbon-flow from protein to TG synthesis, and simultaneously through up-regulation of the expression of a particular set of genes for TG synthesis at proper levels through the actions of SAC1 and SNRK2.2.

Microalgae Cultivation on Anaerobic Digestate of Municipal Wastewater, Sewage Sludge and Agro-Waste

“…Nutritional stress in phosphate limitation is again the more likely reason for increased carotenoid accumulation observed in cells grown in dA30W or dB5W. Lipid and carotenoid increased accumulation could also be related to other stresses which are differently sensed by different microalgae species, such as sulfate deficiency, which was previously shown to increase lipid accumulation in the case of C. reinhardtii [59], or high pH value in the case of dA30W. Increased alkalinity was indeed reported to accelerate lipid accumulation in different microalgae species [60,61] and could be reason for the increased lipid productivity observed in C. vulgaris.…”

Nitrogen-induced metabolic changes and molecular determinants of carbon allocation in Dunaliella tertiolecta

“…Black boxes denote pathway names. A recently proposed C4-like carbon-concentrating mechanism (CCM) in microalgae78798081 is shown to depict an alternative route for carbon assimilation. Neutral lipid droplets found in microalgae consist mostly of triacylglycerols (TAGs), formed by combining FAs and glycerol.…”

Algal Dual-Specificity Tyrosine Phosphorylation-Regulated Kinase, Triacylglycerol Accumulation Regulator1, Regulates Accumulation of Triacylglycerol in Nitrogen or Sulfur Deficiency

“…Microalgae accumulate TAG as a storage lipid in response to a variety of environmental stress conditions, and such a lipid pool could form the basis of a biofuel resource. The model green alga (Chlamydomonas reinhardtii) accumulates significant levels of TAG in nitrogen (N)-deficient (Siaut et al, 2011;Merchant et al, 2012), sulfur (S)-deficient (Boyle et al, 2012;Sato et al, 2014), and phosphate-deficient (Iwai et al, 2014) conditions. Among these conditions, N deficiency is considered as the best trigger of effective TAG accumulation (Boyle et al, 2012;Fan et al, 2012).…”