Cold-sensitive pathway elongates a non-native cis-7-hexadecenoic acid to cis-9-octadecenoic acid in the cyanobacterium Synechocystis sp. PCC 6803

Cellular membranes of the cyanobacterium Synechocystis sp. PCC 6803 primarily consist of glycerolipids esterified unsaturated C18 and saturated C16 fatty acids (FAs). We introduced the Ot17.2 gene from the Mamiellophyceae Ostreococcus tauri into Synechocystis. The cells, Ot17.2+, produced cis-7-hexadecenoic acid (hypogeic acid, 16:1∆7) at the half level of the total C16 FAs, suggesting that the Ot17.2 gene encodes chloroplast-localized C16-specific ∆7 desaturase. To study the effect of the synthesis of a non-native unsaturated C16 FA, we attempted to decrease the copy number of the desC gene for C18-specific ∆9 desaturase, producing cis-9-octadecenoic acid (oleic acid, 18:1∆9) in the Ot17.2+ strain. Surprisingly, the desC gene was entirely deleted in the Ot17.2+/desC- strain, despite the knowledge that the desC gene is essential for survival. We found that the C18 FAs in the strain were unsaturated as in the wild-type cells. In contrast, we could not delete the desC gene completely in the cells expressing the desC2 gene for the C16 specific ∆9 desaturase, producing cis-9-hexadecenoic acid (palmitoleic acid, 16:1∆9). These findings indicate that Synechocystis may synthesize 18:1∆9 from 16:1∆7 via FA elongation, and cis-11-octade cenoic acid (vaccenic acid, 18:1∆11) produced from the elongation of 16:1∆9 may not sustain the cell growth. Interestingly, this strain (Ot17.2+/desC- strain) did not grow and produced little C18 unsaturated FAs at low temperatures. The supply of either 18:1∆9 or 16:1∆7 into the culture of Ot17.2+/desC- supported the growth, suggesting that the lipase activity involved in the FA salvage and elongation system might be severely sensitive to low temperatures.