Photosynthetic microalgae are currently the focus of basic and applied research due to an ever-growing interest in renewable energy resources. This review discusses the role of carbon-unit supply for the production of acetyl-CoA, a direct precursor of fatty acid biosynthesis and the primary building block of the growing acyl chains for the purpose of triacylglycerol (TAG) production in photosynthetic microalgae under stressful conditions. It underscores the importance of intraplastidic acetyl-CoA generation for storage lipid accumulation. The main focus is placed on two enzymatic steps linking the central carbon metabolism and fatty acid synthesis, namely the reactions catalyzed by the plastidic isoform of pyruvate kinase and the chloroplastic pyruvate dehydrogenase complex. Alternative routes for plastidic acetyl-CoA synthesis are also reviewed. A separate section is devoted to recent advances in functional genomics studies related to fatty acid and TAG biosynthesis.

The oleaginous green microalga Lobosphaera (formerly Parietochloris) incisa accumulates high amounts of arachidonic-acid-rich triacylglycerols (TAG), in particular under conditions of nitrogen starvation. This photosynthetic organism is of great interest for studying the mechanisms responsible for storage lipid biosynthesis and the deposition of long-chain polyunsaturated fatty acids (LC-PUFA) in TAG. In this work, we report on cloning a complementary DNA (cDNA) for the putative L. incisa glycerol-3-phosphate acyltransferase (GPAT), whose deduced amino acid sequence features distinctive motives found in those mammalian and Arabidopsis GPAT isoforms that have been implicated in TAG biosynthesis. Temporal analysis of LiGPAT expression in the course of nitrogen starvation showed a positive relationship between changes in the transcript level and patterns of fatty acid production. When expressed in Arabidopsis leaf mesophyll protoplasts, the green fluorescent protein (GFP) fused to the C-terminus of LiGPAT localized outside the chloroplasts in agreement with its predicted extraplastidial localization. Based on an in silico analysis of a deduced amino acid sequence and on similarity to other GPATs participating in TAG biosynthesis, LiGPAT was expressed in the green model microalga Chlamydomonas reinhardtii in order to confirm the predicted function in a heterologous microalgal system. Overexpression of LiGPAT resulted in an up to 50 % increase in the content of TAG on a cell dry weight basis as compared to the control in the stationary phase culture without negative impact on growth parameters. Total fatty acids and TAG of the transformant lines featured an elevated level of oleic acid (18:1 n-9) and a concurrent decrease in C18 PUFA. Additional studies on the acyl substrate preference of LiGPAT are required.

SummaryImpaired carbon precursor supply through cpPDC is detrimental for TAG biosynthesis in Chlamydomonas reinhardtii under conditions of photoautotrophy and nitrogen starvation.

Studies on multisite phosphorylation networks of cyclin-dependent kinase (CDK) targets have opened a new level of signaling complexity by revealing signal processing routes encoded into disordered proteins. A model target, the CDK inhibitor Sic1, contains linear phosphorylation motifs, docking sites, and phosphodegrons to empower an N-to-C terminally directed phosphorylation process. Here, we uncover a signal processing mechanism involving multi-step competition between mutually diversional phosphorylation routes within the S-CDK-Sic1 inhibitory complex. Intracomplex phosphorylation plays a direct role in controlling Sic1 degradation, and provides a mechanism to sequentially integrate both the G1-and S-CDK activities while keeping S-CDK inhibited towards other targets. The competing phosphorylation routes prevent premature Sic1 degradation and demonstrate how integration of MAPK from the pheromone pathway allows one to tune the competition of alternative phosphorylation paths. The mutually diversional phosphorylation circuits may be a general way for processing multiple kinase signals to coordinate cellular decisions in eukaryotes.

No abstract