Accumulation of acyl plastoquinol and triacylglycerol in six cyanobacterial species with different sets of genes encoding type-2 diacylglycerol acyltransferase-like proteins

Recently, acyl plastoquinol (APQ) and plastoquinone-B (PQ-B), which are fatty acid esters of plastoquinol and plastoquinone-C respectively, have been identified as the major neutral lipids in cyanobacteria. In Synechocystis sp. PCC 6803, Slr2103 having homology with the eukaryotic enzyme for triacylglycerol (TAG) synthesis, diacylglycerol acyltransferase 2 (DGAT2), was identified as responsible for the synthesis of these plastoquinone-related lipids. On the other hand, TAG synthesis in cyanobacteria remains controversial due to the low accumulation level within cyanobacterial cells together with the high contamination level from the environment. In this study, to quantify more precisely and elucidate the relationship between the accumulation of neutral lipids and the presence or absence of DGAT2-like genes, plastoquinone-related lipids and TAG were analyzed directly from total lipids of six cyanobacterial species with different sets of genes encoding DGAT2-like proteins belonging to two distinct subclades. The results showed that the synthesis of these neutral lipids is highly dependent on clade A DGAT2-like proteins under the culture conditions used in this study, although accumulation level of TAG was quite low. In contrast to APQ highly abundant in saturated fatty acids, the fatty acid composition of TAG was species-specific and partly reflected the total lipid composition. Gloeobacter violaceus PCC 7421, which lacks a DGAT2-like gene, accumulated APQ with a high proportion of C18:0, suggesting APQ synthesis by an unidentified acyltransferase.