Effect of nutrient modification on lipid level in cyanobacterium synechocystis sp. Pcc 6803 overexpressing aas gene

Abstract

In this study, Synechocystis sp. PCC 6803 strain overexpressing aas gene (Ox-Aas) was successfully constructed via single recombination. The putative acyl-acyl carrier protein synthetase (AAS) is a key enzyme in fatty acid recycling process encoded by slr1609 or aas gene. The amino acid sequence of Synechocystis AAS contained two highly conserved regions similar to acyl-ACP synthetases of Arabidopsis thaliana and Synechococcus sp. PCC 7942. The effects of physiological factors including the nutrient deprivations and acetate supplementation on total lipid and unsaturated lipid levels in Ox-Aas and wild type (WT) were investigated. Under normal BG11 condition, Ox-Aas growth and pigment contents were slightly lower than WT. However, the oxygen evolution rate of Ox-Aas cells was higher than that of WT. Our finding demonstrated the higher lipid accumulation of Ox-Aas compared to WT at all growth phases including log (L), late-log (LL) and early stationary (ES) phases. The highest level of total lipid of both strains was obtained under L-growth phase of about 15.3% and 22.2% of CDW in WT and Ox-Aas, respectively. Nevertheless, the unsaturated lipid levels of both WT and Ox-Aas strains were not significantly different. On the other hand, all nutrient-modified conditions including BG11-N, BG11-P, BG11-N/P and BG11+acetate, caused slight decreases of both cell growth and pigment contents in Ox-Aas and WT. Under BG11-P, the accumulation of total lipid level of Ox-Aas was increased to 27.3% of CDW at L-growth phase. Interestingly, the unsaturated lipid content of Ox-Aas strains was enhanced by 1.25 fold (about 3.5% of CDW) within 8 days of treatments under BG11-N/P condition. For acetate supplementation at both 20 mM and 40 mM concentrations, growth and pigment contents of WT and Ox-Aas showed no differences. In contrast, the total lipid and unsaturated lipid contents were significantly decreased with treatment time when the acetate concentration was increased. On the other hand, Ox-Aas strain had an increase in the expression of plsX, aas and phaA whereas the reduced expression of accA was observed when compared to those of WT. The BG11-P activating effect was found in WT with the increased expression of accA, aas and plsX whereas its inhibitory effect was found in Ox-Aas strain with all gene transcripts influenced by internal homeostasis balance. Altogether, the increased lipid accumulation of Synechocystis sp. PCC 6803 was achieved by overexpression of aas gene. The increase of lipid content in this overexpressing strain under normal growth condition was also observed under phosphorus-deprived condition.