Background
Long-chain free fatty acids (FFAs) are a type of backbone molecule that can react with alcohol to produce biodiesels. Various microorganisms have become potent producers of FFAs. Efforts have focused on increasing metabolic flux to the synthesis of either neutral fat or fatty acyl intermediates attached to acyl carrier protein (ACP), which are the source of FFAs. Membrane lipids are also a source of FFAs. As an alternative way of producing FFAs, exogenous phospholipase may be used after heterologous production and localization in the periplasmic space. In this work, we examined whether Rhodobacter sphaeroides, which forms an intracytoplasmic membrane, can be used for long-chain FFA production using phospholipase.
Conclusion
We showed that long-chain FFAs can be produced from metabolically engineered R. sphaeroides heterologously producing PLA2 in the periplasm. The FFA productivity was greatly increased by high-cell-density culture in two-phase culture with dodecane. This approach provides highly competitive productivity of long-chain FFAs by R. sphaeroides compared with other bacteria. This method may be applied to FFA production by other photosynthetic bacteria with similar differentiated membrane systems.
Results
The recombinant R. sphaeroides strain Rs-A2, which heterologously produces Arabidopsis thaliana phospholipase A2 (PLA2) in the periplasm, excretes FFAs during growth. FFA productivity under photoheterotrophic conditions is higher than that observed under aerobic or semiaerobic conditions. When the biosynthetic enzymes for FA (β-ketoacyl-ACP synthase, FabH) and phosphatidate (1-acyl-sn-glycerol-3-phosphate acyltransferase, PlsC) were overproduced in Rs-A2, the FFA productivity of the resulting strain Rs-HCA2 was elevated, and the FFAs produced mainly consisted of long-chain FAs of cis-vaccenate, stearate, and palmitate in an approximately equimolar ratio. The high-cell-density culture of Rs-HCA2 with DMSO in two-phase culture with dodecane resulted in an increase of overall carbon substrate consumption, which subsequently leads to a large increase in FFA productivity of up to 2.0 g L-1 day-1. Overexpression of the genes encoding phosphate acyltransferase (PlsX) and glycerol-3-phosphate acyltransferase (PlsY), which catalyze the biosynthetic steps immediately upstream from PlsC, in Rs-HCA2 generated Rs-HXYCA2, which grew faster than Rs-HCA2 and showed an FFA productivity of 2.8 g L-1 day-1 with an FFA titer of 8.5 g L-1.