Acylated plastoquinone is a novel neutral lipid accumulated in cyanobacteria

Abstract

Although cyanobacteria do not possess bacterial triacylglycerol (TAG) synthesizing enzymes, accumulation of TAGs and/or lipid droplets has been repeatedly reported in a wide range of species. In most cases, identification of TAG has been based on the detection of the spot showing the mobility similar to the TAG standard in thin-layer chromatography of neutral lipids. In this study, we identified monoacyl plastoquinol (acyl PQH) as the predominant molecular species in the TAG-like spot from the unicellular Synechocystis sp. PCC 6803 (S.6803) as well as the filamentous Nostocales species, Nostoc punctiforme PCC 73102 and Anabaena sp. PCC 7120. In S.6803, the accumulation level of acyl PQH but not TAG was affected by deletion or overexpression of slr2103, indicating that acyl PQH is the physiological product of Slr2103 having homology with the eukaryotic diacylglycerol acyltransferase-2 (DGAT2). Electron microscopy revealed that cyanobacterial strains used in this study do not accumulate lipid droplet structures such as those observed in oleaginous microorganisms. Instead, they accumulate polyhydroxybutyrate (PHB) granules and/or aggregates of alkane, free C16 and C18 saturated fatty acids and low amounts of TAG in the cytoplasmic area, which can be detected by staining with a fluorescent dye specific to neutral lipids. Unlike these lipophilic materials, acyl PQH is exclusively localized in the membrane fraction. There must be DGAT2-like enzymatic activity esterifying de novo-synthesized C16 and C18 fatty acids to PQH2 in the thylakoid membranes.