Background: Monounsaturated fatty acids (MUFAs) are the best components for biodiesel when considering the low temperature fluidity and oxidative stability. However, biodiesel derived from vegetable oils or microbial lipids always consists of significant amounts of polyunsaturated and saturated fatty acids (SFAs) alkyl esters, which hampers its practical applications. Therefore, the fatty acid composition should be modified to increase MUFA contents as well as enhancing oil and lipid production. Results: The model microorganism Escherichia coli was engineered to produce free MUFAs. The fatty acyl-ACP thioesterase (AtFatA) and fatty acid desaturase (SSI2) from Arabidopsis thaliana were heterologously expressed in E. coli BL21 star(DE3) to specifically release free unsaturated fatty acids (UFAs) and convert SFAs to UFAs. In addition, the endogenous fadD gene (encoding acyl-CoA synthetase) was disrupted to block fatty acid catabolism while the native acetyl-CoA carboxylase (ACCase) was overexpressed to increase the malonyl coenzyme A (malonyl-CoA) pool and boost fatty acid biosynthesis. The finally engineered strain BL21ΔfadD/pE-AtFatAssi2&pA-acc produced 82.6 mg/L free fatty acids (FFAs) under shake-flask conditions and FFAs yield on glucose reached about 3.3% of the theoretical yield. Two types of MUFAs, palmitoleate (16:1Δ9) and cis-vaccenate (18:1Δ11) made up more than 75% of the FFA profiles. Fed-batch fermentation of this strain further enhanced FFAs production to a titer of 1.27 g/L without affecting fatty acid compositions. Conclusions: This study demonstrated the possibility to regulate fatty acid composition by using metabolic engineering approaches. FFAs produced by the recombinant E. coli strain consisted of high-level MUFAs and biodiesel manufactured from these fatty acids would be more suitable for current diesel engines.

Biosynthesis of phloroglucinol, an important polyketide compound, has been achieved in engineered Escherichia coli. However, the high level of inherent phloroglucinol toxicity impairs the growth of the producing strains and reduces the efficiency of this bioprocess. It was hypothesized that suitable protective agents might reduce the inhibitory effects. In this study, 4 protective agents including betaine, mannitol, proline, and trehalose with concentrations at a range of 0–1.0 g/L were examined. The results demonstrated that mannitol and trehalose could protect the producing strains against high level of phloroglucinol stress and boost the biosynthesis of phloroglucinol in the recombinant strains in a dose-dependent manner, while betaine and proline showed little positive effect. Mannitol and trehalose (1.0 g/L) increased the phloroglucinol titer from 0.75 ± 0.01 to 0.89 ± 0.03 and 0.96 ± 0.04 g/L, respectively. The protective mechanisms of mannitol and trehalose were also investigated. It was found that the toxicity of high concentration of phloroglucinol was related to reactive oxygen species and that exogenous mannitol and trehalose could increase the activity of superoxide dismutase while showed little effect on catalase.